I have 2 appletv 4 with the same problem. The video starts to stutter after a while. More precisely to reproduce:
Open plex. Video plays perfect.
Put appletv to sleep
Re-open plex (it was opened already and cached)
The video will run mostly smooth but will start to stutter every once in a while.
This is somehow related that plex was running in the background and is not freshly started. The video stuttering occurs to movies and live TV dvr. There is no transcoding going on.
As a workaround:
Kill the plex app (double tap the screen icon and flip up)
Restart the app
Video will run perfect.
Concluding, there must be a bug that leads to the stuttering when the app was run once in the background. Can this be investigated Pls?