On the Shield TV, photos continue running when Plex is backgrounded.
I only noticed that the photo slideshow was still running because I was browsing my apps when the audio from an uploaded mobile video started playing out of nowhere.
It took me a moment to realize that it was the photo slideshow still running in the background (you can’t see the slideshow anywhere, but you can hear the audio from videos playing).
Reopening Plex did not resolve this problem either. The slideshow just kept playing on a phantom thread with no indication from the Plex app that anything was playing.
The only way to stop the photos from playing is to go into the settings and Force Stop the Plex app.
*Technical information available upon request if needed to troubleshoot this bug.