Which version of keys or Kessy do you have, with some versions of kessy (where can keep key in pocket) you can walk up to the boot and it unlocks without unlocking the doors. (but I don't know if Fabia mk4 has that)
So really only need the middle button to lock it up again, but most would just use the lock button.
@munkebo66 can I suggest you check the locking settings in infotainment first, go into, car, then settings, scroll to the option with locking and tick or untick what you want. It might just be set up differently to how you expect and not a fault. Only if then still have a problem contact the dealer.
Actually if it is new car, spend a few minutes and go into every setting in the list and set it up with your choice. Mine was delivered with a safety system off, either the front radar or blind spot monitoring (I forget which now), but if I hadn't been through list wouldn't have known.