2012 Prius III Door Lock Actuator Woes

In January 2018 the driver's side door lock actuator failed. The symptom was that when you locked the car with the remote key or touch-sensitive locks on the door handles, the locks sounded like they first locked and then immediately unlocked, without the confirmation beep. The actuator was replaced at a cost of about $430, 70% of which was the cost of the part.

About 2 weeks ago (July 2018) it started exhibiting the same symptoms again. This time after some experimentation I determined that it was the passenger door that was failing to lock. Here's a list of symptoms

• If the passenger door is unlocked when you attempted to lock the car with the remote, door handle sensors or trunk lock button, you get the lock/unlock sound and no confirmation beep. Only the passenger door remains unlocked; verified by putting the key fob out of range and trying each door handle.
  
• If the passenger door is already locked, either because it was never unlocked or because you manually locked it using the rocker on the inside handle, then locking happens normally.
• Curiously, if you lock the doors using the driver's side lock/unlock button on the armrest, locking also happens normally, without exhibiting the symptom.

Clearly, having both locking mechanisms fail like this on a 6 year old car with less than 55k miles indicates either a design or manufacturing problem. I've opened a case with Toyota.

The third bullet above is really puzzling. What is different in lock actuation between the remote/smartkey method and the driver's side armrest lock button? I would expect that all "lock everything" commands, regardless where they originated, would activate the same locking mechanism. Why does one method fail while the other functions correctly?