Similar experience, LG OLED65C9 with the latest web OS, latest plex app on the LG, Last Plex Server running on Synology NAS.
On the LG I start the plex app, start playing a music play list and then start a photo slideshow, shuffling my library of photos. Works great for about two minutes and then the LG screen saver comes on. Moving the remote solves the problem but this is annoying.
Understand the need for a screen saver when there is a static image displayed. Scrolling photos are far from static and am trying to figure away around this issue.
