I just spotted this and am experiencing the same thing today. Except in my case, it is only Teams that are being provisioned themselves via Graph API that are not working with the GET /teams endpoint. To replicate, use the Graph Explorer and https://graph.microsoft.com/v1.0/teams POST request (with required body) and then this Team cannot be found via the GET request, nor can it be found in /me/joinedTeams.
I'm assuming there must be backend issues if something that has worked for months, suddenly has stopped and on the same day as someone else here.