I have seen a few people mention this in this topic Fast scrolling up but I am not sure the devs have seen what really happens when trying to scroll up.
I have made a video, esentially on the Fire TV you can fast scroll down perfectly, but when trying to fast scroll back up every 10-15 channels or so it jumps to the grid view (favourites) button, pressing up does nothing, you have to press either down or right to get the selection back to the grid you can then fast scroll again for another 10 channels or so.