I got the UI upgrade to the new OS version of my Fire TV cube (Fire OS 22.214.171.124 OS7734/2039) this morning. After the upgrade the Twitch app now will go to the screensaver while watching a streamer and not interacting with the cube for a time. I was watching a Twitch stream last night and this was not happening before I got the notice about the new OS and UI change this morning. I use my Fire Cube to watch mostly Twitch and SlingTV, and I can also tell you that SlingTV does not have this problem and turns off the screensaver like an active program should since I had SlingTV on in the background all day and had no screensaver problems.
I tried to report this using the Amazon support chat and got the scripted advice to reboot the cube. So I did that with no affect. This seems just to be a problem with the Twitch app in the updated OS; it just doesn't or can't turn off the screensaver like it is supposed to after the upgrade.
It would also be great if there was some REAL person or email address to send bugs to. Trying to report a bug through Amazon's horrible support maze is not a productive exersise to say the least.