The auto trim goes almost to the max value when it should have quit at the same time the AP and A/THR have.
It should have quit based on what? Your opinion?
In the absence of airspeed to switch the pitch mode to the Low Speed stability(i.e., Direct law -trim becomes manual only), the pitch mode remained in g-load demand. As the g-load reduced due to the increasing descent rate when the stall happened, the flight controls atttempted to bring the g-load back to the commanded value in the only means it had: more elevator. When the elevator is held at other than neutral, the stab trims, and so it went.