The app is now locked into portrait orientation and the directions inputs for navigation won't respond. The cursor is stuck on the button that used to be the settings menu, but with that menu gone, I can't access it to make any changes. It has basically become unusable with a remote.
I managed to find a workaround, but its not a permanent solution. I have a bluetooth keyboard with touchpad I can pair to my tv, so I was able to access the settings menu by starting a video. There was no way to change the list rotation or re-enable remote control inputs. But after exiting the video, the settings menu button reappeared and remote control navigation re-enabled. Restarting the app also reset the orientation detection to landscape.
I'm not sure why all this worked, but it seems to have made MX Player functional on Android TV again.