We have a user who can not join a customers organization from his android phone.
Said user already has a Teams Education Account and is part of an organization. If he clicks on the link he receives from us, he is asked to accept the invitation and whether he wants to switch the organisation. If he presses okay on both, he receives an error message "guest user was not redeemed" (sloppy translation from German "Gastbenutzer nicht eingelöst").
However, in the teams admin center, I can see that he already is in the group he was invited to and in his teams app on the phone, he can see the customers organisation under the "accounts and organisation" menu. Also I can see the user in the Teams admin center.