Some of our users see this message at random with connected SharePOint Online Doc Libraries:
"You are no longer authorized to sync %LibraryName%"
%LibraryName% has been deleted from your PC
See the attached screenshot (Dutch)
After this message the library isnt in sync anymore but the folder is still there (default behaviour, i guess) but the users dont realise what happend and start working with their offline files which results in a file difference locally vs online.