I don't know much time you have in tactical jets, but you have enough chutzpah to have come to aviation that way.
Actually it is zero and I got into professional aviation through kind of integrated course. I'm an aviation buff that got very, very lucky to have both pictures taken by me and pictures of me posted on airliners.net. Don't concentrate on who is behind the nick but rather on what is written in the post.
The performance of the autotrim system in this corner of the flight envelope likely does not meet certification requirements.
Given your previous post, I think you might be mistaking the description of flight test method with certification requirement.
THS running full nose up is a potential serious hazard and needs to be guarded against better.
It did not run away, it was commanded to trim by FC computers.
How to do that is one for the engineers (design type not wrench type) to puzzle out, but judging from what has recently transpired with the type, it may save two or more aircraft in the future.
Or kill two or three in the process if all implications of having such a system don't get well thought out and crew gets caught out by losing autotrim when they don't expect it.
During this time period the SS was in the average NU, but never full NU
Neutral is 1G demand, full in clean is 2.5G demand. assuming linear sidestick response (I don't know whether it's precisely linear but during my time on bus it sure felt like it), half stick would be 1.75 G demand - impossible to meet below 1.38 Vs so with all air data rejected as unreliable and inertial reference available, both elevators and trim would try to meet it no matter what. That's why they ran to their limits.
The THS can be limited in movement by using the hand wheel in the cockpit.
Except "don't do that", I can't find official reference what would happen if you try it. When we tried it out on the 320 sim, it simply ran to position demanded by FCCs when released.
There is another method of stopping the nose-up trim: push the stick forward.
Has it something to do with the priority button?
Yes. FO SS inop means capt's priority button pressed and held an v.v. There's also latch out if priority button is held long enough but given the traces, it wasn't the case here.