Are the cool & warm side hides identical? If not, say if the "cool" hide is smaller (cozier) or if there's more substrate in that one (making it feel cozier) that may also be why he prefers it. Just something to consider, & the reason we go for at least 2 matching hides. In general, we offer hides that aren't overly-large, with only one doorway that's also not huge, so the hides feel private.
Another reason he appears to prefer the cool side could be that the warm side is actually hotter than the temp. you've shown. Yes, your temps. sound fine- as long as they were taken correctly. What I mean by that is we're not seeing your set-up- or how deep the substrate is, & what kind of heat you're using (UTH?) etc. If the substrate is too deep over UTH, the heat can build up & actually be much hotter than is comfortable or safe for the snake in the "warm hide" so you may want to double-check your temperatures on any surface the snake may contact to be sure that's not the reason your snake is staying in the cool hide. Substrate is often pushed to the side & dug under by snakes, so when checking temps. you need to check that way also- not just on the top of substrate.
But understand too that it may be nothing wrong at all. I'm just explaining some other possibilities.And since you've only had this snake a short time, please be subtle about double-checking temps. so as not to stress him. A new snake that's eating is great, & there may be nothing at all that needs adjusting.