I've read several related posts on this issue, but none sounded quite the same as what I'm seeing. Running Illustrator 25.2.1 on Mac/Catalina, saving to a Mac configured (afp) server.
Randomly (non-reproducable) Illustrator will create .tmp files in which changes are saved. Adjustments in the original file are not stored in the .ai, but in the .tmp. This happens when Cmd-S, but also when Cmd-Shift-S and overwriting the original .ai. Cmd-Shift-S with a different filename works without issues. At times the .tmp file cannot be deleted (only after disconnecting the connection with the server), sometimes the .tmp file can be deleted. However, deleting the tmp file results in loss of (adjusted) data. The .tmp files can be renamed to .ai.
This issue started occurring after updating our Macs to Catalina and Adobe CC2020. Save (and Export) in background are unchecked, we tested with both Autmatic recovery checked and unchecked, also tested Optimize File Open ... with both checked and unchecked.
Bug, Import and export, Performance, Sync and storage