One Microsoft 365 group disappeared from Outlook client

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.

What should I try next?