I mentioned this during the tvOS 14.3 beta but now that's it's officially released, I can confirm the bug still exists in Channels.
When trying to organize favorites in the "On Now" grid, the highlighted channel loses focus. So you can't move them around easily, you have to reselect the channel again before every move. Try it (with tvOS 14.3, released today) and you'll see what I mean. In tvOS 14.2 the channels would move consistently and intuitively, making organization easy. Now, they don't.