OK, so I found out what it was and where it came from.
3 weeks ago I printed these dust covers for the X-Y motors:
https://makerworld.com/en/models/170634#profileId-187309
(they have since been taken down for very good reasons, but I had bookmarked them…)
After reading this post https://www.reddit.com/r/BambuLab/comments/10jvjrj/comment/j5rz43b/ , I realized that the P1S was homing without end-stop switches (even my old Ender 3V2 had some!)… And the covers where basically creating an offset when pausing in the back left corner.
So if you ever find a shift, it is probably because your machine loses its reference. So either you have to add a homing action of X-Y after every time it goes to the back left corner or find out what is creating the shift.
It still doesn’t explain why my plate was scratched, but now I understand how critical it is for the motors to move freely, without missing steps.
Thanks to everyone.