Personal experience:
Problem: Autosave is "Off" if a file is opened from Finder. Autosave is "On" if a file is opened from the file menu in Excel/Word/Powerpoint.
Autosave Working: Late 2015 iMac running macOS Catalina (10.15.4), Office apps from office.com (16.36), and OneDrive v 19.232.1124.0012 (standalone).
Autosave NOT Working: 2020 Macbook Air, identical software. And yes, all the Office autosave, macOS extensions and full disk access boxes are checked.
Hopefully the team can get this figured out for good. I suppose it's also possible Apple keeps patching whatever method they're using to send Finder "meta-data" from OneDrive to the Office apps.