I have the exact same issue for a few weeks, also using a Synology DS918+ as PSM and streaming video to the Samsung Plex app. As soon as embedded subtitles are selected (no matter if transcoding, streaming or direct playing is chosen), the image is scrambled. Overlayed ones are mostly out of sync…
This is obviously rendering Plex useless, as I’ve reverted to playing on PC and using casting to TV. Another post talks about the same issue, so hopefully this gets addressed quickly.
