Are you streaming remotely? Otherwise, Plex Relay shouldn’t come into play.
What information is showing in the “now playing” tile for that playback on the Plex server dashboard? Specifically, is the stream listed as Local, Remote or Indirect?
https://support.plex.tv/articles/200871837-status-and-dashboard/
Did you already have a look at the related troubleshooting support article?
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
There’s also a guide with some more details on how to troubleshoot remote access issues.
