About 15 years ago, when I was still flying PG and NG 737’s, we had a de-icing procedure that required us to place the stabilizer full nose down.
Including using the trim wheels, all the way to the stop.
It was said that this was necessary, so that the de-icing fluid could run down the aft end of the horizontal tail structure.
This procedure was abandoned later, and repaced by a neutral stab trim setting. So that fluid would not accumulate between the stabilizer and elevator, and subsequently freeze up during flight, impairing elevator authority.
Why am I telling you guys this? Well...
Very often, as I placed the stab trim full nose down with the trim wheel, the following would regularly happen.
It regularly was VERY hard to return the stab trim. INCLUDING using the trim wheels. You had to yank and pull the trim wheels quite a few times so that they would snap free, and the it was usable again and trimmable.
And this was at the gate/de-icing platform. Airspeed was zero.
If this “snap free” hardware problem turns out to be the issue here, than...Boeing is in for a hardware modification. That requires a totally new certification.
I suspect the MCAS is capable of driving the trim wheels all the way forward and capable of causing this same “snap free” lockup as I experinced regularly at the gate.