I’ve seen this. Sometimes its on the bottom and sometimes on the right or left sides if the video aspect is letterbox.
As a test try and disable both direct stream and direct play on the plex for playstation app. I’ve found that the ONLY way to get decent playback using a ps4/ps5 is to disable these forcing plex to transcode. Not great but works.
I’d started a discussion a while ago on this but it was auto closed after being ignored. Since disabling the above we’ve not had any playback problem, including this green line.
Continuing the discussion from PlayStation 5 Stuttering (not buffering, stuttering) when Direct Stream / Direct Play is enabled: