An oddball problem with an oddball fix and some workarounds.

Autosave is a highly useful macOS feature that prevents losing incremental changes to a document you’re working on between the periods when you press Command-S or choose File > Save. As you work away, the current state of the document is updated temporarily. When you close the file, changes are applied even if you don’t use the Save command or keystroke.
…
Read full post here:
https://www.macworld.com/article/3518898/how-to-fix-the-cannot-be-autosaved-bug-when-saving-documents-in-macos-apps.html