Had that same problem on wife's 02 Optima a couple of years ago.  Had to
have the dealer replace the on-board door lock controller.

Rich

I live in the U.S. and do not know what a Cerato is.  But I do own a Kia
Sedona minivan.  I have noticed that when I use the keyfob, I have only a
few seconds (I don't know - maybe 10, 15, 20, something like that) to get
one of the front doors open before the van re-locks.  And if I open
something other than a FRONT door (sliding side door, back hatch, etc.), it
will still re-lock everything else.

Don't know why this is or why they would program it like that.  The only
thing you can do is to not use the fob until you are much closer to the
vehicle, and then even if you want to open up the hatch/trunk, quick pull
the front door open and re-close it again.  That keeps it from re-locking,
at least it does for me.

Yes, it's a pain, but not enough for me to take a trip all the way to my
dealer and have it in the shop most of the day, waiting for my turn.

One other thing - like most men, I keep my keys in my pocket.  Apparently,
the button is notorious for wanting to be pushed while it is inside the
pocket.