I have noticed that and mentioned it at another forum. I didn't get a reply back, but I'm assuming it has to do with the exposed body and the ab joint having greater range of movement. So I don't think it's an issue anymore than it is on Series 1 bodies, it's just easier to pose him in a way that pops his shirt up.
I've been looking at pictures and watching videos trying to see if there's a chance that the harness is a wholly separate piece that might be glued in place. When Series 1 came out, I bought doubles so I could switch heads and have an Ash from the very beginning of the movie and a Deadite Ash from near the end. So in my continuing quest to have versions of every single look in the movie, I'm considering making a "Workshed Ash" with the chainsaw and harness but without all the battle damage. And if the harness is a separate piece that I could work off the body, that'd be awesome and make the task a lot easier.
From what I can see in pictures the shirt looks the same, like the folds and bunching looks the same. The harness seems to sit on top of the shirt and doesn't really sink into it, so it doesn't appear like they sculpted a whole new shirt piece. That gives me hope.