Is there any way they could be rubbing their head in a tight spot in their vivs? Is there a hole they can't quite fit their heads through or a gap around the lid?
I had a female amel mot look like that when she repeatedly tried to fit her head through an air-hole in a plastic tub. Rubbed the scales right off and they looked just like that, blistered whitish looking. Even now, 2 going on 3 years later her head still shows signs.
I'd watch them to see if they are rubbing those spots raw on something in the viv in an attempt to escape. Other than that, I've no clue.
Let us know what you find.