Looks almost like a burn due to the redness but it's in an odd place. Plus you said the UTH is on a thermostat.
Malnutrition can cause a condition called slipped skin disease due to the lack of vitamin C. Someone else here had a rescue ball python with it. The skin is paper thin and tears easily so the snake needs to be in a very minimalistic enclosure with nothing its skin can get snagged on until it recovers.
https://ball-pythons.net/forums/show...ry-Image-Heavy
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