I've had this a couple of times when the snake has injured itself somehow, and particularly just before a shed. I assume they scrape against something or damage the soft, fresh skin underneath the old one in a similar way, and when they shed the damaged skin is shown. When I had this, I observed it for a bit and when it didn't change at all, I left it as is. Cleared up the next shed.
I only suggest it because I understood scale rot to usually appear on the belly scales (not on the back) due to sitting in damp conditions. I would say this isn't because it isn't red or inflamed, and just appears to be some damaged scales.
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