I depotted around 100 Mac shadows and never had this problem. Did you get alcohol on the eyeshadow? I use 91% alcohol to remove the glue from the bottom of the pan. But still, I haven’t had this problem.
You probably let the eyeshadow on the straightener for too long, or the upper part of the tool was too close to the eyeshadow and the heat sealed it.
Try scraping the top layer, it should be better. What I do when I depot is put a cooking sheet on top of the eyeshadow as well as underneath so the powder gets a bit of protection from the heat as well.