It appears to be a pretty normal thing to me. I am new to keeping frogs but from my understanding they burrow deeper and deeper to maintain humidity? So I guess it's a possibility that he is comfortable. I probably sound like an idiot but that's my 2 cents. I'm sure someone with a better answer will chime in shortly. I wouldn't worry though, my 2 frogs don't burrow completely. And so far they seem perfect. As long as temps are fine and humidity as well, I'm sure your froggie is perfectly fine