Outlook Calendar can display details but scheduling assistant can't even show free/busy after EXO migration

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.

Could you try to remove calendar permission and re-add the permission then check if this issue continues?

I have found a blog on Users can't see free/busy information after a mailbox is moved to Office 365, you could refer to and it may be helpful to you.

In addition, Folders can be accessed cross forest in many scenarios, but they are not fully supported by Microsoft as outlined in Permissions in Exchange hybrid deployments

There is an easy way to point you most likely in the right direction: run SARA from exrca.com. Check if the routing address uses your domain.mail.onmicrosoft.com and that the enterprise trust set up correctly. This is most likely not a (mailbox) permission problem.