2010 Prius. Will not auto-lock doors

It seems strange that the dealership couldn't figure out your issue; instead of playing wack-o-mole with parts replacement. I'm pretty sure Tech-stream, under BCM (Body Control Module) will display each door actuator state - along with door lock button state. Simply manually toggle each door lock - the one that doesn't change states is the defective unit. Ditto with the electronic lock switches.
If your BCM - ECU is bad, you'll likely get a CEL. Has your car been under water, flood damaged or your doors has ever been worked on? There are control rods back there that may have simply fell off. A defective aftermarket car alarm CPU may also be messing with your lock system. Another possibility is door sensor issue, If the ECU thinks that one or more doors is ajar, it may not activate the electronic locks.

Hope this helps....

 

Similarly (I think Sam mentioned this already), if the ECU does activate the locks, and then does not see all of them arrive fully at the locked position, it then unlocks them all. (At least, that was what my Gen 1 did.)

 

You are assuming that the dealerships service department is competent......and that they are not lying to you.
NEITHER of those things might be true.

Pay close attention to the action and sound of all the doors as you lock them.
If it really IS one of the actuators, it eventually will get bad enough that they won't lock "manually" either.

Note: The post above from BioMed is one of the best I've seen lately.
Pretty much covered it all.

 

Just had a brain-fart..... Have you checked the child lock-out/over-ride switch on your rear doors? It may be in-between position; lock vs unlock, confusing the BCM or actuators.

Report back if that's the problem.