My Plex stopped working recently, the Docker seems to fail to complete startup. I came across your post and checked my setup and I'm seeing the same thing as you are. The app I have installed shows linuxserver/Plex but the one shown when searching the store shows lscr.io/linuxserver/plex. Did we miss the memo?
Looking at the More Info page for my installed conatiner: https://hub.docker.com/r/linuxserver/plex/
The docker-compose file references the image at lscr.io/linuxserver/plex:latest
So it seems something may have become disconnected with the image URL change.