I would suggest to change back to mulch and throttle down the humidity to 55 to 60%.
Reason for why I think that the substrate might be the culprit: Substrates are being discussed on just about every forum and every keeper and breeder is trying hard to provide the very best for their critters. That's a tough thing to do, because fact is that some snakes have different needs. The metabolic rate of a ball python is not just affected by temperature (although some BP's will attempt to hibernate even with perfect warm temps in a tank, like in your case), but also by exposure to certain chemicals/minerals/proteins in their environment. Aspen and cypress have a different ratio calcium/magnesium uptake and used as a substrate may affect the snakes metabolism.
Sad, but true, there are many research projects about mineral uptake in plants, but to my knowledge there are none that provide much information as to their effect on our snakes.
If you decide to change back to the mulch, give your baby a couple days of rest and then try to feed again.
Best of luck!