Get the undertank heater out ASAP as it’s an easy way for your frog to burn itself while burrowing. It’s better placed on the side of the enclosure if used at all. I’ve temped heat pads at over 140 degrees and that’s a hassle you don’t want believe me. As far as burrowing goes some will burrow at different times. It can be triggered by dryer colder conditions. In the wild these frogs will burrow and seal themselves in a coat of hardened mucous to trap body moisture. This is called aestivation. I’ve had many individuals burrow over the years and the best way to bring them out is to add more water to the substrate. Can you describe the terrarium type, substrate, light cycle, ambient room temperatures! That may help me and other members in advising you further.