It is happening Ignasis, Had this happen to me very often. Playing on a private Server running Siptah, zero mods.
You just have to move around certain Terrain(Rocks, Trees, Walls, stairs, foundations) with your thrall following. At some point you see him sticking to it and not moving anymore.
I have the thrall on attack everything all the time, so he stops from time to time and say “hello” to a rocknose,rhino. When he teleports back is the point where he might get stucked.
The most of stucking happends inside vaults, it happends when the animal/npc does his full Combo and Knock down/back my thrall. The thrall does not recover until i bring the animal/npc back in his melee range, or the animal/npc dies.