Apparently, the mosfet has a leaky gate. Current of 0.1 uA takes less than usual voltage (-26 V), and only at 10 uA (50% duty) gate threshold began to change.

The process that followed is quite interesting. It ended with an almost killed mosfet. Gate-to-source now measures about 140 KOhm.

Interesting is that it started very slowly, then accelerated, and then slowed down and stopped.

This seems to be a slightly dodgy mosfet, so I won't do any further analysis of it. UPDATE: no, it is an all-right MOSFET.

UPDATE: attempt to recover it with +35V finished it off. This MOSFET is now a dead short across all 3 pins.