A TRACON Controller reported an aircraft on approach descended below their assigned altitude and flew below the Minimum Vectoring Altitude.

Narrative

I issued Aircraft X a descent to 065 as he was inbound from the east requesting the TACAN XX approach at ZZZ. Aircraft X read back; 'Descending to 065'. After this a commercial jet from the south was checking on; and subsequently asking questions about how busy I was; and if I needed to give him a short cut for the sequence he could take that. The pilot was trying to be helpful but was interrupting my scan. I was responding to his questions and continuing my scan when I noticed Aircraft X at 060 in the 065 MVA east of City X. I issued a Safety Alert to Aircraft X and he advised he was climbing back to 065. As Aircraft X was joining the TACAN inbound to ZZZ he advised me that he had been struck by lightening. I asked if he required any assistance and he said no. I believe maybe the lightening strike and him descending below the MVA were related but I'm not sure. There was precipitation in the airspace but there was none depicted within around 10-15 miles of Aircraft X so none was issued.KC135s from ZZZ need to ensure that they are descending to the proper altitude or advise that they are taking evasive maneuvers if that was the case.