Here is the answer from Airbus customer services:
QUESTION:
Planned Answer :
I have been asked why on landing rollout the managed speed starts to increase passing 45knots ground speed. Why does the manage speed start to increase passing this speed when decelerating on landing?
ANSWER:
The managed speed on landing rollout is calculated by the FG part of the FMGC and is called "Approach speed target" (APP SPD TGT).
The formula is:
APP SPD TGT = max[VAPP ; VAPP+GROUND SPEED MINI]
* The VAPP comes from the FM part of FMGC and is memorized by the FG in G/S mode below 700ft. So the increase on the approach speed target must be due to the ground speed mini.
* GROUND SPEED MINI = TOWER WIND + TAS - VGROUND
With:
- TOWER WIND memorized in approach below 700ft, limited to [-10kt ; -100kt]. - TAS frozen below 60kt.
So, if VGROUND is still valid below 60kt, GROUND SPEED MINI will mechanically increase below this speed (TAS being frozen and VGROUND decreasing).
When VAPP+GROUND SPEED MINI becomes greater than VAPP (at 45kt in your case), the APP SPD TGT starts to increase.
So, the APP SPD TGT increase at the end of rollout should be due to the air speeds (TAS) becoming invalid.