We had an issue after adding a new domain to our tenant using the Hybrid Configuration Wizard which removed the URI and URL strings in the availability settings in Exchange Online. That was resolved but 2 days later free/busy from Exchange Online to on premise Exchange users broke again.
We had to add a new SAN in the certificate for Exchange and suspect that the Federation Metadata did not properly update. One of my colleagues found this article which pointed us to the problem and advised us how to resolve the issue.
In short running:
Test-FederationTrust -UserIdentity yourcredentials showed the issue.
To resolve my colleague ran:
Get-FederationTrust | Set-FederationTrust –RefreshMetadata which refreshed the metadata and resolved the free/busy issue from Exchange Online to Exchange on premise users.