When does any Pilot LOOK to see what the THS is doing in normal cruising flight ?
The fact that is only, or largely, in one direction MAY not be noticed, if as in this case the Pilots were trying to get the aircraft to behave.
Perhaps a slight finger pressure on a SS should have had a compensating pressure from the ball of the PF's right hand.
BUT this was possibly something which he had never done much, at cruising level.
Some have suggested that there should be a limit to the AUTOMATIC adjustment of the THS to, say, FIVE units. Others may know better.
(PS My employers had to record the B707 trim readings when sending data to Boeing for performance monitoring. in the 1970s.)
LT