Ugh. I’m in the same boat here. Facial animation from Voice2Face and then tried applying some default motion to the body from the Metahuman demo project (which eerily matches my spoken dialogue). Head is detached from the body. As soon as I switch Animation Mode, it temporarily “snaps” into place, but that is just an illusion - as soon as you play the sequence, it’s back.
I tried manually offsetting the head animation, but that just gets nasty when the body starts to move and the head stays put.
I feel like there must be a way to “lock” the head to a bone on the body, but can’t for the life of me figure out how.