I am pretty sure there is some documentation on Planner SSO, but I am not able to find it. Do anyone know if planner is compatible with SSO? I could still see planner authentication while adding it to Microsoft Teams. Is it the same behavior like everyone is facing while adding Planner to MT?
It is not a problem of Planner, but how Planner (and it also happens with other services) is integrated in Teams...and in regards of your question, the answer is Yes, normal behavior is to be asked to type Planner Credentials when you have started a fresh session in Teams