First of all, thanks to all the ones who replied with useful suggestions
I can confirm that the problem is only the liner, not the whole top-case.
I had looked in the LT maintenance manual before posting here, and it does mention the screw
as a fixing point for the liner, that's why I was surprised to see it moving up so much. I tried to see if it's broken, but I don't have a picture of a "good" one so I can't tell if mine is broken or not.
The liner is slightly moving up also in the area where you should have the two clips
next to the latches, but again it's hard to know if those clips are in place, since I don't know how they should look like.
Unluckily the pictures in the maintenance manual are far from being clear. Any picture of the two parts above would greatly help
I guess that the only way to know what's going on, is dismount the whole inner liner and check everything: screw/bolt, clips and velcro.
PS: I was aware of the "mod" that you can apply, by adjusting the longitudinal position of the case, the whole procedure is shown in the maintenance manual. Anyway, since my passenger is still happy with the space she has, I won't touch anything for the moment.