maybe you can help me out. So one of my collegues was invited as a Guest to the customers tenant. Unfortunately he can't even switch to the customers tenant because the option is not shown next to his profile picture. We tried with the client app and the web browser. We even ask the customer to look up in his AAD to ensure that he is there as an external. So this wasn't the problem.
He even received a mail that he was added to a new Teams.
Was the users UPN/Email address changed after being added to Teams? This can cause the issue. For example, I had an address of firstname.lastname@example.org and then about 6 months after using Teams this was changed to email@example.com. Whenever I get added to someone else’s tenant using the second, the same behaviour happens. When the other organisation adds the first before the change (Chris.firstname.lastname@example.org) it works without issue.
Did he accept the invite on the proper account and not a Microsoft account? They can follow these steps: https://docs.microsoft.com/en-us/azure/active-directory/b2b/leave-the-organization like they were going to leave(but don't leave obviously). They should see the other tenant here if they accepted it on the proper account. If this looks good, then just give it a little bit of time, it should pop in eventually, it took 30min to an hour for it to pop in for my guest tenant earlier, not sure how longs it's been there.