I'm reporting this in the Fire TV channel because all of my Channels DVR clients are Fire Sticks. I have no way of knowing if this bug presents itself on any other platforms.
It's pretty simple. If you're browsing the channel Guide and you land on an entry that extends beyond the right edge of the grid, you can't scroll right. See attached video (first half of video shows proper scrolling, and then I move up one channel and you see that scrolling is broken).