Some of the gliders I fly do the same: They do not droop ailerons in full landing configuration but do in intermediate settings. There the reason is aileron effectiveness at high angle of attack and prevention of a tip stall.
If you droop the ailerons it yields more lift but the angle of attack at which the airfoil stalls is decreased, which might give you a more violent stall. And if you suddenly deflect the aileron down further the flow could separate and cause "the wrong wing to go down". To avoid this airliners are often fitted with vortex generators on the outboard section (e.g. Boeing 737) but that was probably a no-go for the clean A350 wings.
Regarding droop vs. flap setting, the FCOM probably has the answer for that. Couldn't find it online yet. A380 does 5 degrees when flaps reach position 1+F or greater, A320 does 5 degrees when flaps reach 1+F or greater so it might be similar with the A350