@RWA
So if the THS responded to the 'nose-up inputs,' why did it not also respond to the nose-down ones, and instead remain at full nose-up?
Good question.
To be specific I think your question is how do we reconcile this contradiction in BEA report:
At 2 h 10 min 51
...13 degrees nose-up in about 1 minute and
remained in the latter position until the end of the flight.
and then
At 2 h 12 min 02
...PF made pitch-down inputs. In the following moments, the angle of attack decreased, the speeds became valid again and the stall warning sounded again.
IMHO the BEA are making a generalization saying THS stayed at +13 deg, or else how do we explain the pitch down inputs followed by a change in AoA. In which case this appears to be rather sloppy writing or a poor translation from the french original. What they should have said "...remained [mostly] in the latter position...".