Hi developers,
this is a longstanding bug:

With facing pages documents on EVERY document page there is a zone where jumping of objects will happen.

I call this the jump zone. All objects on a page that will jump are clearly inside the page's area. The bug has nothing to do with rounding errors where object edges are at the edge of a page.

The objects should not move if pages are added or removed, if numbering of pages is changing. Nevertheless they do. Always. Unless, the zone has a width of zero.

Below all the details.

What is a jump zone?
If horizontal values for left and right margins differ, there is a BUG that in effect forms a zone on the page.
I call this zone JUMP ZONE because objects that touch the zone with their edges could jump horizontally.

When will objects jump?
1. When a left-hand page becomes a right-hand page OR a right-hand page becomes a left-hand page.

1. When a left-hand page becomes a right-hand page and later when a right-hand page becomes a left-hand page again.

This could happen within a single InDesign document open.

WARNING:
It could even happen to InDesign documents that are not visibly open.
That's with InDesign documents that are part of a InDesign book file.
The book file's feature with auto-numbering pages can do this even if no document is visibly open.
The damage is done if auto-numbering took place AND the book file is saved.

How large is the jump zone?
The height of the zone is the height of the page.
The width of the zone can be calculated:
inner margin MINUS outer margin

How large is the jump?
The width of the jump zone.

Where is the jump zone positioned?
Positive value: The zone is at the spine of the page.
Objects that touch the zone jump towards the spine.

Negative value: The zone is at the edge of a page away from the spine.
Objects that touch the zone jump away from spine.

What versions of InDesign are affected?
All versions CC up to InDesign 2020, Mac OS X and Windows.
InDesign CS6, Mac OS X and Windows.

Possibly also InDesign CS5 and CS5.5, Mac OS X and Windows.
I did not test this.

How can I avoid jump zones?
Make right and left margin values the same!

Are there workarounds if I cannot avoid jump zones?

1. Anchor objects
   that touch the jump zone to text frames that do not touch the jump zone.
   Nested objects will not jump if the parent objects are not touching the jump zone.

2. Group an object
   to another one that is touching the top or the bottom edge of the page.
   Tests are showing that objects will not jump if they touch the bottom OR the top of the page.
   ( OR both, bottom and top edges of the page .)

Other things to consider about moving objects?
The following is independently of jump zones:
Objects could move to other pages if their edges touch exactly the horizontal edges on pages in facing pages.
Depending on the version of InDesign this could also happen with objects touching the spine.
Reason: Rounding errors.

What can provoke all this?
1. Add or remove an odd number of pages
so that left-hand pages become right-hand pages and vice versa.

1. Renumber start pages of sections so that left-hand pages become right-hand pages and vice versa.

Note:
Every document has at least one section by default.
The start of that section is the first page in the document.

More details at Adobe InDesign Forum:
https://community.adobe.com/t5/indesign/bug-in-book-s-update-numbering-feature/m-p/10774458#M166408

InDesign 2020 documents attached here.
See screenshots and further comments at Adobe InDesign Forum:
https://community.adobe.com/t5/indesign/bug-in-book-s-update-numbering-feature/m-p/10774596#M166420

Download link to all test files on my Dropbox account:
https://www.dropbox.com/s/duov096cu2q481l/Jump-Zones-Explained-Facing-Pages-Documents.zip?dl=0

Regards,
Uwe Laubender

( ACP )

PS: Why does InDesign Uservoice not permit to attach files with suffix .indd ?
Message: "Invalid file type: .indd files not allowed."
Isn't that rather absurd?