What is going on is, Unreal optimizes everything, which is why it can run so well on so many systems. Calculating animations can be a generally slow task, so, by default, any animated objects that are “occluded” (technical term for hidden from view) stop getting animated, and most of the time they also stop getting rendered altogether (which is called occlusion culling). There is a way to disable this behavior, but I don’t know how, which is why I’m posting this as a comment.