From what you've described, your setup is darn near perfect and you're doing things right. As far as the spot, it looks like it could be very mild scale rot.
At any rate, I'd say just continue to treat it like you've been doing, and so long as you don't notice it getting worse, then don't worry. It will correct itself in a shed or so, as long as you keep the husbandry good like you have been.
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