Demystifying Hybrid Free/Busy: what are the moving parts?

thorbug , I am not sure about Teams client and how the query / display is done, what I can suggest for the hashes (f/b issue) is that you try F/B from OWA and see if you see the hashes there too (you can use F12 Network Tab and look at the Response for GetUserAvailability) or you can try F/B test on exrca.com https://testconnectivity.microsoft.com/tests/FreeBusy/input when this issue happens in Teams. If the issue is reproducible on the Exchange /OWA client side, then check the error message and check part 2 of my blog, especially the PDF with common errors and resolutions. Those are mainly applicable for Hybrid Free/busy (between exchange on-premises user and exchange online user) but suggestions might help.  If issue is reproducible only in Teams and not Exchange /OWA, then I suggest you open a support case with Teams.