The Plex app is limited to 80 Mbps when playing HEVC video (60 Mbps for H.264). The limit is for the entire media file - video + audio + subtitles - not just the video track. It is a Samsung limitation and applies to all apps, not just Plex.
The bitrate shown in Plex is an average for the entire file. If the media stream bursts above 80 Mbps, you will experience buffering.
If the video track is below the limit, try muxing out unneeded audio and subtitle tracks. This may get you below the 80 Mbps limit.
You can tell if you’re hitting the limit by looking at the Plex client logs.
See this post for details on how to view logs. It also has a link to the Samsung documents showing the bandwidth limits.:
