This bug is very easy to reproduce (and very annoying). While watching a movie on the Shield TV, press the center button on the remote to bring up the menu. Press the right button on the remote and highlight the fast forward icon and press the center button to fast forward. The menu will disappear after a couple of seconds. Any further presses of the center button will fast forward the movie. The expected behavior in this case would be to pop-up the menu again. Note, the same issue happens after rewinding.
I am using the original Nvidia shield TV and remote control, not the 2017 version. I am using the latest version of Plex (6.14.1.3753) which I believe was released today. This issue also exists in the previous Plex version.
Note, it is possible to workaround this issue by pressing the up button to access the menu instead.
