Hi Jill,
The Assign Team functionality is newer than the (original) Assign Person functionality, and (I suspect As Designed) behaves differently.
When a person is assigned to a Task (or named as the Project Owner, etc), Workfront also “puts” the user into the ProjectUser data table behind the scenes, thereby driving the “Projects I’m on” connection.
When a Team is assigned to a Task, however, it is more of an invitation for a member of the Team to “pull” that work (by accepting it via Work On It, aka Start Work), at which point Workfront then assigns their user, puts them in ProjectUser, and considers it a “Project I’m on”.
Regards,
Doug