Please I have a colleague who reported this issue to us.
We have problems accessing the Exchange calendar from Microsoft Teams for users with the mailbox still on Exchange 2016 on-premises. The Exchange hybrid connectivity is set up and we synchronize all users with AAD Connect. A user migrated to Exchange online can see it’s a calendar from Microsoft Teams.
Because the Hybrid server is configured on another URL, we’ve changed the TargetSharngEpr on the OrganizationRelationship and IntraOrganizationConnector to match the Hybrid server URL.
After this change, the Free/Busy started to work from a migrated mailbox to an on-premises mailbox.