A Microsoft file hosting and synchronization service.
Sharing what fixed this for us (so far), in case it helps others.
We had the exact same symptom on several Macs in our organization: Office files in synced SharePoint libraries, opened from Finder, showed "Saved to my Mac" with AutoSave off, while opening them via File > Open > Sites worked fine.
We were already using the standalone OneDrive (not the App Store version). What resolved it was a complete removal and clean reinstall of OneDrive:
- Quit OneDrive.
- Ran the reset script from the app bundle (OneDrive.app > Show Package Contents > Contents > Resources > Reset...Standalone...command).
- Moved OneDrive.app to the Trash.
- Removed all remaining OneDrive-specific items in ~/Library (Application Support, Caches, Containers, Group Containers, FileProvider), the OneDrive LaunchAgents/LaunchDaemons and receipts, plus the OneDrive entries in Keychain. Careful: do NOT delete the UBF8T346G9.Office or UBF8T346G9.ms group containers, they are shared with Office.
- Restarted the Mac.
- Reinstalled the standalone OneDrive from Microsoft's download page (not the App Store) and re-synced the SharePoint libraries from the browser.
After this, files opened from Finder are recognized as cloud documents again and AutoSave is on.
Current setup: OneDrive standalone 26.168.0830.0006, macOS 27.0.1, Apple Silicon.
Make sure everything is uploaded before unlinking or removing OneDrive, as files that haven't synced could be lost.