The fix for this issue is now available in the latest update of InDesign – that is, InDesign 15.0.2.
Please update your InDesign application to get this fix.
Adobe InDesign team
An error occurred while saving the commentChris Scott commented
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.)