I used to be notorious for ripping apart my cages if I couldn't find my frogs. If he continues to do it, check the conditions in the tank, i.e. the humidity, temperature, etc. If everything appears normal and he continues to eat, then don't worry too much. I know Phyllomedusa can deal with drier temperatures, but it could be humidity driving the frog to burrow down into the presumably moister soil.

Best,
Alex