That makes sense on the Xbox using a small buffer and would explain the trouble then. I wonder if Chrome and Plex Web is the same type of behavior. Going to have that user install the new Windows app and see if it persists. Recommended a couple of normal Xbox users to try a different device when they have a problem. Have a FireStick user complaining that every couple of minutes it skips back 3-5 seconds but I'm gonna write that off as a FireStick issue as every user on my server that uses a FireStick reports weird trouble and I generally recommend they replace it with a Roku.
I wonder if it's possible to force a transcode in the Xbox app as I have one user that that is all they have. They would probably have to manually set their quality to something other than what the source is to force it.
Edit: The Xbox Plex settings actually have a checkbox for Allow Direct Play. I unchecked it and the affected file now forces a container transcode which prevents it from using the Xbox buffer and therefore doesn't cause the buffering.