I use an HD Homerun Extend with an ATV 4K. Sometime within the last couple of weeks, live TV via Channels stopped playing. I can stream content via the ATV, and access the HD Homerun via their IOS app, so I'm concluding that it's the HD Homerun <-> ATV <-> Channels connection. While troubleshooting, I noticed that I have the Extend's transcoding turned off.
By "stopped playing", I mean that when I select a channel via the Guide, the first couple of frames show, then it just stops. If I wait a few seconds and hit the forward button on the remote, it moves to the new spot in the timeline. The ATV Stats overlay shows that video is being processed... but nothing changes on the screen.
The ATV is updated to the most recent system software, and Channels is up-to-date.