Undead mobs burning underground in superflat in pitch black darkness

So, for some reason, every time I see mobs spawn in this cave I have blown out (using a TNT cannon) they seem fine, but when I approach them they burst into flames, and even after destroying a few blocks, and even replacing them, they still spontaneously catch fire. This could just be something with my world, but it's still kind of a pain to deal with. Keep in mind that where the mobs spawn is pitch black, covered up, and also deeply underground.