It isn't really a bug though this is a very common task we'd like to make easier! Right now what you're probably hitting is a combination of the fact that elements are always positioned from their top-left and that Hype will do a little bit of convenience work to keep items in the same position when growing a group, but this doesn't extend to animations.
The solutions is that you need an animation of the inner elements to go in the opposite direction to keep their position the same. There's a lot more discussion with some nice graphics illustrating what to do in this thread: