While all our end user mailboxes are still on our on-prem Exchange 2016 environment, I have moved a few room mailboxes to EXO. When I view these mailboxes in the calendar view in Outlook, I can see the details of the appointments as I have reviewer access on those calendars. However, when I create a meeting invitation and I go to the scheduling assistant and add that meeting room, I get a message that the free/busy information could not be retrieved.
I double checked permissions, a security group that I am a member of and that has reviewer access to the calendar of the mailbox. The organizational relationship has also been verified, I cleared the cache in my Outlook, tried with an entirely new profile in Outlook and even see the same in OWA indicating it is not a client issue.
Most topics I can find online point in the direction of not having correct access rights and point to needing reviewer level of higher to see free/busy, but they also talk about no access to the calendar in general which, as I can see from my calendar view, is not the case. This is limited to the scheduling assistant. I have even set both the Default and Anonymous users to Reviewer to see if that might have been the issue, but without success.