Well, buggy file upload as well :-)

trying again.

edit: Tried uploading file again with no luck. :-(

Following up.
I have attached a document where this problem occurs.

The document has two object styles "mainText" and "footnotes".

mainText is simply a single column text frame.
footnotes is a text frame with text wrap and auto sizing.

At first things are pretty normal but after messing around just a little bit – flowing footnote text to a new frame, applying the footnote object style and such – things begin to act strangely.

Changing object styles has no apparent effect.
Changing the size by pulling the text frame handles does not change the flow of text.
Trying to undo a single step completely erases the entire undo history and takes me back to the initial undo step.

I am having the same issue. However, I would point out that the list of items that can be undone seems altered in 2020 version. I opened the same file in 2019, typed into it and in the Edit menu is says "Undo Typing." This is not presented in 2020, it seems to be fixated on "Undo Paragraph Style Options" even if the last action performed was typing, cut, paste, or anything else.
My document is very plain. A single master frame for page after page. But I lost an hour of typing this morning with a single undo. I'm not a software engineer, but it does seem like either the entire stack of commands since the file was opened are being ditched instead of just the most recent one off the top, or else the pointer to the correct event is somehow corrupted (redo also does not work.)

Same problem as everybody else here. Indesign 15.0.1 on iMac 17,1 running Catalina 10.15.2.

As another user suggested it could be linked to auto sizing since this problem just appeared today when I applied auto sizing to some frames.

So, this doesn't fix the problem but it helps to stop that automatic CMD-Z reflex action that trashes your layouts. I've removed the keystrokes from Undo and Redo using the Keystrokes editor, so if I accidentally hit the Undo keystroke, nothing happens. It does seem (to me anyway) that everything works fine until you hit your first CMD-Z. This is a massive issue though, and I'm pretty convinced there's a strong tie-in with the use of auto-sizing boxes.

Any document... Hit command Z and everything done is gone. All of it. Every time.

Turns out it also happens when you try to undo from the menu. I'm working on a 50 page document with character/paragraph styles, 78 linked images, master pages etc. at 3 am and didn't need this headache. It's bad enough that InDesign crashes to force updates.

It doesn't take you back to your last save. I've been saving every time I make a change and I still needed to hit undo and it reverted the document back to a point that I can't put my finger on.

The icing on the cake is now that I've been saving and not undoing I got this error: Adobe Indesign is shutting down. A serious error was detected. Please restart Indesign etc.

Since my last post on the 10th, a lot of people have mentioned auto-sizing text frames as a potential cause/contributor to this problem. That would tie in closely with the documents I'm currently working on for a book layout. I've also noticed a weird delay when applying object styles that include fixed-width frame sizes and auto adjustment to the height only. The frames do eventually resize correctly after a few seconds delay but in the interim, the frames appear to have some sort of inset. To test I opened up an older layout that doesn't use auto-sizing. messed about with it for a couple of minutes, then hit undo and it worked fine without taking me all the way back.

Same issue. InDesign 15.01
Sometime the Undo Command reverse from some couple steps at once, sometime there are no more Undo or Redo at all!

It occurred in this file, along with this other strange behavior, where the text frame does not resize on drag, but acts kind of like a table (There is no table in the frame), where it has its own boundaries. See screenshots.

Don't know if the issues would be related.

My steps to reproduce this issue. Make a large chunk of edits to text. Escape text. Select other text frame. Triple click to select paragraph. Edit paragraph text. Undo because I selected too much text. Luckily noticed that all my edits were lost (thinking back, there were other instances of files with edits I missed that I could have sworn I'd made that must have gotten lost and not noticed).
Mashed redo redo redo. Did some cussing. Bookmarked the infinity publisher site. Came here to vent.

The Undo glitch just happened again today. I just lost a ton of work and will have to switch to the 2019 version while you figure this out. It's happening many times a day.

I've had trouble before with copy and pasting content (photo boxes and text within tables). This time I was typing text within a table and clicked command Z. Whamo - back to my old saved document. Please notify us when this is fixed.

I have previously sent you documents and a video.

This is happening to me when I move graphic boxes around this file (see attached). After working for about 20-30 minutes on it, it reverted to the last saved and I can't redo it back to where it was. Every time it has happened with me, I am doing some sort of cut and paste. It doesn't happen every time but it will happen when I click on an element and option drag it to copy a new version.
https://www.dropbox.com/s/5tccg7ulyi5h7wx/GDS%20038%20Sales%20Data%202019.indd.zip?dl=0

This bug just surfaced for me recently, in InDesign version 15.0.1. I use InDesign daily and have been using whatever the latest pushed out 2020 versions are for I think a couple of weeks. I am still on Mojave.

For me it seems to surface when I’m messing around with auto-sized text frames — not when I’m undoing/command-z-ing, as others seem to be reporting. I have gone back to InDesign 2019 (14.0.3) for now, but now that’s acting buggy in different ways (URLS not showing in Hyperlink panel, can’t quit app). *deep heavy sigh* Adobe, can you give us something that works? Please?

I have had this happen in two different documents in the last 24 hours. One of the instances was working with a table and the other was while moving rectangle boxes around. It seems to be random and won't work again until I restart InDesign.

@Bertrand Richter @fsm Linked text frames with auto adjust are also part of the documents I worked on when this disgusting behavior showed up. This is a key feature to me and as long as there is no reliable fix I have to go on with ID2019.

@Bertrand Richer: Interesting. I work on a document with a lot of text frames that are set to resize automatically. No problems up until now: I deactivated auto-resize for one specific text frame and linked it to another one. I then made some more edits, hit Cmd-Z eventually, and boom – got thrown back to right before when I linked the text frame (placing cursor is active, so it reverted right back to the point when I clicked into the little "+"-icon on the text frame). This text frame also reflows around a placed image (which caused some trouble on its own in the .0-version).

This is still far from being reproducible, but a colleague of mine encountered this bug at a very similar situation as well (linking text frames, cmd-z then reverting back to the moment when linking text frames).

I hope to have found the solution to this bug! Perhaps!!! I disable the automatic sizing feature in Object> Text Block Options> Auto Sizing to all text boxes in my document. In my case, this function was enabled by default.

And if it is true, then it would be this function of Automatic Sizing that would be defective.

I worked this way for a week and the problem did not reappear.

- - -

J'ai espoir d'avoir trouvé la solution à ce bug! Peut-être!!! J'ai désactiver la fonction dimensionnement automatique dans Object>Options de bloc de texte>Dimensionnement automatique à toutes les boîtes textes de mon document. Dans mon cas, cette fonction était activée par défaut.

Et si c'est vrai, ce serait alors cette fonction de Dimensionnement automatique qui serait défectueuse.

J’ai travaillé ainsi depuis une semaine et le problème n'est pas réapparu.

I've not been able to see any definitive pattern to this problem, it's very random. It just happened again a second ago and I did pick up on the fact that my direct selection tool seemed to have stopped working afterward. I couldn't select an image within a frame until a restart. Also, this may be a red herring, but it does seem to happen more frequently when I've applied an Object Style incorrectly, then hit Cmd-z immediately afterward. I'm running 10.15.1.

I have found out the problem occurs mainly when I am working with tables, especially larger ones. When editing a table the undo ALWAYS fails.

I am also experiencing frequent loss of Undo functionality in InDesign 2020. I'm using Windows 10. I haven't noticed a pattern to reproduce it. I'm saving frequently to avoid losing work. When problem happens, I close without saving and quit ID and relaunch. I have tried saving as .idml and resetting preferences to no avail. Doesn't seem to matter if the file was originally started in 2019 or 2020 version.