For anyone googling and stumbling on this, I don’t have a solution to this problem. However, I’ve heard that it’s different for each person, somewhat depending on GPU possibly. Frustrating.
Some people suggest changing temporal sampling and spatial sampling both to the same number. Start with 2, and if that works, go for 4, and so on. I really wish they would fix this, or at the very least have an official “help” or bug report on it.
But yeah, MetaHuman grooms/hair groom jitters with Temporal Sampling enabled is a problem, and isn’t fixed in “5.2”.