Are you able to test without the unionfs mount for the same media? I did that by adding just another mount and adding a test library in plex with just the one specific item / show I want to play.
I basically had the same issue you are talking about and it was able to be detected by the debug log and the player opening and closing the files repeatedly.
If you want to add to the topic I posted in regards to Direct Play, it is here ->
I’ve yet to recreate the issue on my setup as I’ve desperately tried. If you can get the same thing on the rclone mount, I’d be interested in seeing that debug to see if the same pattern shows.
My gut is a particular setting that doesn’t jive with the ATV (4k and regular one) but I can’t figure out which either so have to go through the process of eliminating all the variables.