I’m guessing most will have seen this, but for any who haven’t.
Microsoft in their arrogance have changed OneDrive to make all files off device. It has something to do with Apple deprecating extensions, though MS seems to have cocked up their response.
If you have OneDrive installed, and you open it in Finder (Mac) or Exporer (Win) you should see a cloud icon with downward facing arrow. This indicates that OneDrive has removed the file from your device (without asking permission) and made it available online only.
They have screwed it so badly that if you right-click and select Always Keep on This Device you are likely to get a sync error. The tip is to click the cloud icon and download it BEFORE selecting Always Keep on This Device.
WARNING - if you make changes or especially if you share the file and the other user makes changes, it can revert to online-only without warning.
OneDrive isn’t recommended for iOS syncing though before this should have been safe for PC-PC/Mac syncing and for backup syncing.
Until MS addresses this, I suggest extreme caution.