How is Team Drives backed up?
Spanning Backup accesses Team Drives data through the user account that has the "Manager" or "Content Manager" role on the Team Drive.  We cannot access Team Drives data through Groups or Distribution Lists.

Who can access Team Drives backups?
Spanning Super Admins, Google Super Admins, Help Desk Admins and users that have the "Manager" or "Content Manager" role on Team Drives can access Team Drives backups. If you have disabled access to Spanning for your users, they will not be able to access the Team Drive backups.

Please note that the backup history for Team Drives cannot be transferred from one user to another. If you remove the Spanning license for a Team Drive member with one of these roles, the Team Drives backup history will be reaped in 30 days. Spanning recommends using a service account as a Team Drive member with the "Manager" or "Content Manager" role even if you have other users with these roles to maintain the backup history.