Hi jacsdad,
Thanks for writing back and providing the detailed information to us. Appreciate for your additional test and contacting the Apple support to check the compatibility issue.
As you mentioned the problem doesn’t occur in the previous version of Office applications i.e. 16.29.1, can you please provide the problematic Excel files to us via private message so that it will help us to test in latest Production version and Insider
Fast of Excel application and check the behavior. You can click on this link to provide the problematic files.
Lastly if possible, can you also provide the what are changes made to file before saving and sheet name that changes are made, as it will be more efficient way for us to test in the same way as you do.
Appreciate your patience and understanding.
Regards,
Chitrahaas