As a part of the channel rescan that recently ran, some of the channels I watched no longer exist. Where this appears to be a problem is on the banner (or whatever that thing is called on Apple TV, where Up Next and On Now appears on the Apple TV Home Screen) channels that no longer exist still show up there. And without a way to not make them a favorite any longer, the old channels just hang there with no picture.
Thanks for reporting this bug. The only workaround for now is to reboot the Apple TV