> Is this Microsoft's fault or Synology's fault?

It's 100% on Synology.

> Much of this discussion sounds like an inability to distinguish between first and third party products.

Yeah. Did I explain it badly or something? I thought I made it clear that I'm having an issue with 3rd party backup solutions and that it's possible they're using some shared API from Microsoft, but most people seem to be focused on OneDrive specific issues.