After going back into Plex after the TV has been in standby, when you go back to the home screen the selection box for items disappears so you can't see what you have selected. You can still navigate around and press OK to launch things, but just have no idea what you're clicking on. The only way I've found to fix it is to completely exit Plex with the back button, and launch it again.
How to reproduce:
Watch something on Plex on Samsung TV.
Turn TV off (put it in standby) with the power button on the remote.
Come back later, turn TV on again and launch Plex
Go back to the home screen.
Yellow selection box disappears.