the ap toggle switch also engages auto trim, im pretty sure iv had the problem youre talking about a few times, when you look outside on f2 the stabilators are in pitch up even with full forward stick! the only way ive found to fix it is simply hold nose down trim and itl correct it. look at the trim gauge too thatl read very high.
as far as i can tell it happens when the autotrim is on and i let the speed drop at high altitude, my guess is it keep adding more trim to keep the nose up until you end up in or close to a departure/flat spin.
i dont know if its a bug or simply bad piloting but its rare now im more used to the plane, i got a it quite a bit when i was new to it