I had previously posted a new thread about the bug here: https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/50643335-illust...
Here is an example ai-document that has the bug: https://drive.google.com/file/d/10USCi-VXLvZ0YneUNZUYznUOQyVOCDNh/view?usp=drive_link
Try opening it in e.g. Photoshop or placing it in InDesign, then comapre with opening in Illustrator. You get very different versions.
To fix it, all you have to do is re-save the file, and then it is fine. So it's not consistent, and I've been unable to reproduce it on purpose.
Encountered originally (for a colleague) multiple times on version 29.8.2, and encountered two days ago for me on version 30. Both on Windows 11. Background save was ON for both of us, and the files are located on a network share.
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Since it has happened at least a few times, we now have to manually check every time we save a file if it is actually properly saved (by checking it in InDesign/FrameMaker/etc. to verify the latest changes are included). Not great. But hopefully the tip about disabling "background save" might help (time may tell).