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 commentLiz commented
The same horrific problem—doing a batch of work and then hitting Undo and losing a batch of work, despite having Saved in the meantime—is gone now that I tried the fix offered below, about turning off the auto-sizing in some of my frame objects (and restarting InDesign). Hallelujah! Please fix the bug as soon as possible.