An info about the ATHR logic below 400ft.
The GS mini caries extra speed for gusts and wind change (basically it sustain the same total energy during the approach).
Those extra knots (Actual headwind - tower headwind (the one inserted in the FMGS)) are reduced for about 2/3 below 400 ft (this is done in 25 seconds) in order to land with reasonable Ground speed.
This is why you feel a change in the ATHR reaction below 400. It becomes a little bit sluggish and slow.
An Air France 340 undershoot a runway few years ago in Cayenne (french guyanne) because of a shear on very short final (around 100ft) while in managed ATHR. The low SPEED warning was not even triggered. The aircraft suffered minor gear damages.