I have an user that is in two groups. When using the Outlook web client, he can access email for both groups. One group is a "Private group with Teams" and the other is a "Public group with Teams".
When using his Outlook client, he had access to both groups but this has changed. He now only has access to the "Private group with Teams". The other group is not visible. I deleted and then re-added him from the errant group but this did not resolve the problem.