No reverse on 2022 Yukon

[Yukon-Steve]

Purchased a 2022 Yukon Denali with about 65k miles on it about a month ago
All good for about 3000 miles, now won’t move in reverse. The “R” shows up fine as expected in the dashboard when the reverse button is pressed, and the backup camera goes on, but the car does not move in reverse. The engine seems like it is in neutral. Now also getting an engine light (yellow steady) and an onstar diagnostic message saying transmission problem. Have service appointment Monday. Any guesses? Will post findings.

 

[B-train]

I thought there was some known issues about the valve bodies in the 10 speeds. Maybe that's what they'll find.