We’re actively working on improving our integration with the iOS Files app and have temporarily set it in read only mode. You’ll still be able to open OneDrive files in the iOS Files app and use it to attach files, but you won’t be able to save or edit files in the iOS Files app. We know this is not ideal and are working to improve the experience as quickly as possible. In the meantime, the OneDrive app remains fully functional.
v12.26.1 purportedly restored access:
You can once again make writes to your OneDrive files from the iOS Files app.
This didn’t seem to be working even though my OneDrive app was updated, so I re-installed. The feature works fine with the Share sheet but not with the Save File action in iOS Shortcuts. So on the rare occasion I build a shortcut and need it to save a file, I use the Share action.