A Microsoft file hosting and synchronization service.
My understanding from reading around is that the issue has arisen from Microsoft migrating users from the original OneDrive platform to the SharePoint platform (for consolidation and longer-term support). I suspect that the synchronisation issue exists while shared and sharer are on different platforms - I could easily be wrong. However, if we wait patiently for long enough (for Microsoft to move both shared and sharer to the same platform) normal and proper synchronisation is automatically restored. It was for me / us ...
I'm not excusing Microsoft's poor performance - just offering hope!