My relatives Seat Ateca Xperience? 2021 with KESSY also had the same issue - keys would get locked in the car and boot would immediately lock, despite the "Key in vehicle" warning on the dash - RAC chap said he has had a fair few of these happen (mainly keys in boot). This also happened if you shut the boot after opening using the key only (car locked, opening with boot latch only, or using the remote key). This is in contrast to my 2015 Skoda Octavia (non KESSY), where the boot remains unlocked for 30 seconds post shutting it, if you open with the key button.
The solution, after much fiddling: Using OBDEleven, I found that changing two settings to active in
Module 09 (Central Electrics) -> ZV Heck appeared to fix the issue:
Freigabenachlauf Heck nach Einzeloeffnung ueber HDF-Funktaster
"Release follow-up rear after individual opening via HDF radio button"
and
Nach externer Verriegelung ist Heck noch zeitbegrenzt entriegelt
"After external locking, the rear is still unlocked for a limited time"
Both were inactive, and when changed to active, the boot no longer locks immediately upon closing if you have opened with the key button, or by pulling the latch with the key in your bag (which you then stick in the boot). If you open the boot, then lock the car with the remote, it will lock immediately on closing.
As the RAC chap also advised, we're going to get a bladed key or two cut to keep as spares in different places, at least then if they do ever happen to get locked in the car again we can open it to get in the boot!
Edit: Clarified model (Xperience, not Xcellence).