They're Heated too hot and too long
I've ran into this a lot actually. At first I thought it was because I or them thaw too long. I used to let them sit over night. So I started only letting them sit 4-5 hours in dry air. Then heating them under the lamp minimally. It also happens is you put one under the lamp and the snake doesnt eat so you set it to the side and reheat. The good news is it doesn't hurt the snake. And somehow they still consume most of it with the exception of some of the wet stuff. Aside from it being kind of disgusting and smelling kind of bad it doesn't really seem to hurt anything. Just air thaw for 5 hours or so and heat once. For the most part you'll be fine.