This affects ONLY the ATT TV app.
When playing a channel with Dolby 5.1, using any trick play or pause causes the audio to switch to PCM 2.0
Exiting the program (switching channels) and going back, and hitting resume, you get DD 5.1 again until you use trick play again.
FireTV stick does not have this issue with the ATT App.
EDIT: It also does NOT affect the Roku Premier or Roku Ultra (2019 version), so I have just plugged an external Roku into the AVR for now.