Glad to help. I should also note that while the Channels team does contribute to the mpv codebase, I don't know if all of their additions make it upstream; therefore the player in Channels may differ from other apps. Just take that into consideration while troubleshooting.
(Another point is to try with a fresh slate: first see if closing every app and then hard rebooting the AppleTV makes a difference; and secondly going scorched earth to see if a hard reset of the device makes a difference. The first is easy, but the second is a commitment to troubleshooting. I'm just trying to put out all the options to try and rule out odd extraneous issues.)