Were there any re-shoots after Carrie passed away? A stand-in (real or otherwise) makes perfect sense for certain shots either way. No studio pays actors to lie down all day anymore than they do for shots where a character is seen from a distance or from behind.
Certainly nothing on the level of a Rogue One, but every movie ends up requiring pick up shots to some extent. No way of knowing if any of them had to do with Leia, and that doesn’t even matter honestly because, even if there was, it’s basically impossible to tell from the finished film.