Hey, first thing I noticed that could be contributing to him burying himself is your humidity cant have that much fluctuation through out the day. You need to try and keep up around 70-80%, anything lower then 70 is starting to get on the dry side for them and anything over 90% for long periods of time you can start to create bacterias in you tank. It needs to be a steady 70-80%. Your temps at 65 are extremely cold for him and being exposed those temps for to long can cause serious damage to kidneys and other stuff internally. The lowest it should go at night is 75f. From the dry and cold conditions your frog has been in he may be trying to aestivate or he may have become ill. I would bump his day time temp up to about 82f and don't let his night temp drop under 75F, get the humidity at a constant 80% and see if this makes him resurface. And next time he does come up or you dig up if you could snap a pic of him that would be helpful to determine what is up with him. Does anything about his appearance seem off to you when you dig him up?