2017 Rear door latch / actuator problem

SOLVED, mostly. See below.

Original post:
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I got a used 2017 Prius a couple years ago and only just discovered the rear door doesn't lock.
It sounds like it locks, but someone from the outside can pull the handle, which causes it to open halfway, then pull the handle again, which opens the door. From the inside, the passenger is always locked in until someone lets them out from the outside.

I thought it was the child safety lock, but moving that up and down and trying various things doesn't change the behavior.

Has anyone else had this problem, and am I gonna have to go through the misery of prying the door panel off and removing the lock mechanism to investigate further?

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"SOLUTION":

On closer inspection, it looks like this only happens when the rear passenger tries to open the door while locked, but only moves the lock switch halfway. That puts the system into a state where the electrical actuator can't lock it or unlock it, and it stays sort of half-locked, which requires two pulls from the outside to open the door, while the inside release can't do anything.

The solution is to simply move the lock switch above the door release to either the full lock or full unlock position. Then everything works fine again. It's simple, but it's not obvious when you look at the switch.

It's not worth investigating *exactly* what's going on within the mechanism. I just need to pay attention when a passenger exits the rear doors before they get unlocked electrically.

 

Yes, I've only noticed it on one side--the curb side, which is used much more often.
But I think I figured out mostly what's going on, which I'll add to my first post.