I’m also experiencing the same issue on my Xbox One S, and even on the original Xbox One. I’ve had a large number of Plex Server software upgrades, it also doesn’t matter whether it’s running natively on Windows or in a Docker on unRAID (Linux).
It happens to me for H264 and H265 content, 720 and 1080, and that isn’t with any hardware transcodes running, and when hardware transcodes run, it also happens (using an Nvidia 1060 Ti) or Intel QSV on a 6700K Skylake CPU.
I’ve had Plex Server running on an AMD 6-core CPU with 16 GB RAM and the Nvidia 1060 Ti (Windows) and in a Docker on unRAID with an Intel Core i7 6700K with 16 GB RAM and the onboard HD 530 iGPU.
I don’t get stuttering issues if I use my Samsung Galaxy S9+ or S20 Ultra, no jittery playback on my desktop or any other device… it seems specific to the Xbox One. The hardware should be MORE than capable of smooth playback so what is going on?