Citroen dispatch rear door lock issue?

Hi all,
I have a 2014 Citroen Dispatch MK2, with an annoying issue.

The rear door will not lock with the central locking.
All other doors lock ok, but when you try the rear door with the handle, it opens and then unlocks all other doors.

If you lock the van using the key lock on the rear door, it locks all the doors ok, and unlocks with the central locking unlock activated.

I have replaced the locking unit with a supposed good unit. And no different.

Any ideas please.

Regards
Paul