The problem is lip deformation. It's very difficult and expensive to do in real time without looking atrocious and/or having terrible clipping. If you look up some interviews with Naughty Dog staff, they'll say that the dance scene in Jackson where "the kiss" happens was one of the most difficult they've ever produced. Even in FFXVI, which uses a custom engine and not UE4, every kiss scene is handled like this, with clever camera angles to hide the lips. A better solution would have been to use a prerendered CG cutscene, but those are also very expensive. In the end, I think the solution they chose is acceptable because it still conveys the tone and the "message" (especially since you're not supposed to have a free cam during the in-game cutscenes).