I was just about to post about the exact same problem. Z11 Pro Max hijacks system audio. No sound from MyTv3 nor from other sources connected to my TCL Roku TV.
Have to unplug the Formuler, and restart the TV to recover sound. Tried every troubleshooting procedure I could find (multiple times) and no luck. About to pack up my unit and return it for a refund.
Sure would be nice to get a resolution, because it’s a great box otherwise.