The big thread about this issue ( OneDrive Synced folders cannot detect content change ) points to it being a problem outside of Opus, at either the operating system level or something that's interfering (e.g. antivirus or something else which hooks Windows APIs).
For some people, the change notification API isn't sending events about the OneDrive folder to Opus, and it seems that it can be temporarily "fixed" by a completely unrelated program requesting the same events in parallel. (Which should have no effect on what happens to Opus's notifications, unless there's a bug outside of Opus.)