Strange lock key problem?

I have had from new. Low 125,000 miles now. It has been a trouble-free vehicle with one weird exception. Even the dealer gave up: First of all, both my key fobs work normally. Door lock switches inside work normally. Vehicle doors correctly lock automatically at 15MPH. But, if I try to unlock the vehicle using a key in any of the three key locks (driver door, passenger door or tailgate) the following happens: the door(s) unlock but within a split second, with key still in the lock, the key in my hand is forced back by the actuator leaving all doors locked. If I then try to unlock several times then one time, I will get lucky, and door(s) stay unlocked, and key can be pulled out normally. It has been this way since just after the warranty expired. Obviously, it cannot be an actuator problem with all three actuators developing the same fault while the vehicle was basically new. I have never heard of anyone else with this problem. What on earth is causing it? Is there some faulty timer at work here, or a bad relay?