Concours77, as the stab seems to trim to full ND at a normal rate, I don’t think there is a mechanical failure of the stab jackscrew.
What perhaps happens is a breakout of the control yokes just before the dive. FDR shows a difference in the applied control force on the yokes. I believe when elevator breakout occurs there is sudden loss of elevator effectiveness. This would cause a sudden bunt over, with a wtf moment and the pilots stopped any sustained opposite trim.
what do you think?