I am new to Plex for Xbox One so apologies if this is a known issue (I did check the known issues post from @Moussa )
I experience consistent frame skipping when movies Direct Stream to my Xbox One X. Most of my movies require Direct Stream because the Audio format is not compatible with Xbox One X it seems. When the occasional movie uses an audio format that is compatible and I'm granted Direct Play I get buttery smooth 24p playback.
When on Direct Steam a frame or 2 will skip every 10 or 15 seconds. I'm sure most people wouldn't notice but once you see it you cannot unsee it.
Xbox One X attached to local plex server via ethernet.
Plex has Display Rate Switching turned on
Happy to attach logs if you let me know how/which ones to upload.