You should keep him at the proper temps no matter what- they will regulate themselves when needed, or they'll get RI.
The other possibility is if you aren't using the same hides on both sides and he finds the cool side hide more secure somehow.
I've tried switching the hides too and no matter what I do he stays on the cool side, but like I said he rarely comes out to roam so I'll just leave it and let him do him lol