Have you tried checking the Tv for software updates, then rebooted it by unplugging it from power for 5 minutes?
I’m curious as to how you’ve connected you CONNECT:AMP to your TV set in the first place, Do you have analog RCA cables from an output on the TV? I assume there’s also some lip sync delay. What TV set is this, and what are the outputs labeled that the Sonos is connected to?
It seems odd that the TV is switching back to the TV’s audio. Since an analog connection has no way to feedback anything about what the Sonos is doing, the change must be somewhere in the TV. If this were a Sonos Amp, and the connection were HDMI, then CEC might be a culprit, but there is no such connection on a Sonos CONNECT:AMP.