I only know how to investigate problems from logs - unless it is a clearly understood and known issue
see
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
I suggest you have a controlled test after enabling debug logging and restarting the server and just do the test - no other streaming / playback on the server. Also enable roku app network logging beforehand with logging level in settings set to DEBUG
Provide full details of the steps followed and the times and outcome and download the server logs - See https://support.plex.tv/articles/200250417-plex-media-server-log-files/
