I'll echo what Cheesenugget said. CHEs work great for ambient heat but they do suck your humidity out of anything you put them on. For me part of what sold me on PVC was the fact that I could use a RHP instead of a CHE so I could avoid the problem you're having right now. If I were you I'd look into an alternative, especially in the size enclosure you're using.
I have zero experience with Underground Reptiles, so I can't speak to the quality of the enclosure, but just at first glance I would point the finger at your CHE for your issues. Especially if you're using a typical on/off thermostat. I found that even though your CHE is controlled by a thermostat, it still heats up to wide open to get to whatever temp you set it to, and it stays hot for awhile even after the thermostat turns it off. I had much more luck switching to a proportional thermostat, but the most luck came from just swapping to something else all together.
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