I'm California, and this week I've had the AC on to about the same temperatures. There's not much humidity (though I haven't measured it), it's a closed room with no windows and Tombo doors. Although I did sometimes turn off the AC, and it might raise to 75 degrees or something before I turn it back on. It's good to know you had done the same but nothing happened.
My first thought was, I got this MISB from AnoToys in Philippines, and maybe they have higher humidity. I got it within a week from them, then I posed it in that pose right after I opened it. Maybe if I had let it "air dry" for a bit, the material might have stuck together. Taking precaution now with my JL WW and not put anything on her for a bit.
I actually got an Armored Batman MISB from them as well, and the butt area had a "mark" on it from sticking to the plastic tray. It looks a bit shinier than the other parts of the butt. That's why I'm guessing it's how they stored it.. but then again, Armored Batman is super old sitting in box.
That's what I thought. I felt the material and it seem like much more resistant to stretching, and people have complimented on the increase in material quality, so I left it like that. Like I mentioned above, maybe air drying before posing might have avoided this. And also not leaving it in extreme pose for "so long".
I don't I really forced it, but it definitely wasn't super super slow. I think it kind of "snapped" out of that locking position? I don't quite remember. I think some parts of that will "heal", but the parts that peeled off will not I don't think.