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I don't think it's that bizarre really, it is something that can happen sometimes, if you give them the ideal (from their point of view!) cage with lots of substrate & places to hide away, good food so they don't have to constantly come out searching in the hope they might get nice treats they can become reclusive, they just sort of go feral!
I'm sure you'll get very mixed opinions on this but it might be worth considering keeping him in the smaller cage for a while again & do some intensive taming, get him out consistently each evening & have some sessions in a safe play area or make a play pen & keep that up until he becomes more tame again, then maybe try putting him back into the larger without so many places to hide away or too much substrate then if he still comes out & remains tame & interactive you could gradually change his bigger cage until you find something that really works for both him & you.