I checked this out on my Mara, you can see there is more to this bug than just the butt area. If you look at the legs of my char they are miss colored. This reminds me quite a bit of the Relaxed Vestment's bug. If its the same type of issue good luck ever seeing this fixed.
I have not seen at all whatever is going on in your screenie but I'm skeptical that this is just a bug. The rear of the bikini is larger than it was, the cloth goes higher up the back than it did before. Before, it was low enough you can actually see the beginning of....what's the medical term?...anyway, before the cloth stopped just just below the beginning of the butt crack. Now it does not as now it goes higher up the back. Looks like a failed photoshop job to me so now the question is did they break the models beyond repair now?
This little whatever it is has messed me up in game LOL. My toon has a back flap of sorts covering the rear but it's not long enough to completely cover the discoloration. It actually looks like there's a light shining down the inside of the back flap and that's as good at having neon flashing arrows pointing at my butt constantly. And then, and then there are those up-skirt camera angles in cutscenes that actually zoom in on the effected area.
Wish we can get some kind of official statement on this. It's taken me off stride in game. When I start a character I want to finish that character, not simply go play a different one. I'm having a problem playing this character with flashing neon arrows pointing at its butt.