Add owners also as members (AAD Group) in the initial creation dialog from the Teams Admin center
The Microsoft Teams Admin center allows the creation of new teams. In a UX update the admin is now able to add multiple accounts as an owner in the initial dialog. Using this method results in a not fully setup membership. Due to the underlying AAD group it depends on the M365 Group service used if the accounts can access its content. The initial admin based team creation only sets the users as an owner of that group. The accounts are not added as a member. Any other M365 service will set the users as members and as an owner if specified by the user. For Teams and SharePoint there is no problem, due to the implementation of the permission check. But many other services will rely on standard (A)AD membership checks. For example: Planner will check the membership based on "members". A newly created Teams with such an owner configuration will not be able to access planner. I created a Microsoft Ticket (#20365134) because I think this is a bug, but the ticket was closed as this is by design :(
This problem only happens if you use the Teams Admin center (what you should do if admins create Teams and not the user through self service) and you add the owner in the initial dialog. The normal membership management in the admin center will always require the admin to add the user as member and then promote them as an admin.
For me this is clearly a bug. So please go vote to have this fixed.