For example, where does it say that validation is even called upon leaving focus of the combo's lineEdit?
Here (QLineEdit):
When editing is finished, either because the line edit lost focus or Return/Enter is pressed the editingFinished() signal is emitted.
And since the validator is applied during text input, and because finishing the text input through focus change is part of the input, the validator should be called as result of focus leave.

I put a breakpoint in my validate method to see if it's called at any point during my UI/programmatic interaction. I'm not seeing any calls so I'm guessing something in my wiring is wrong.
Ok, that is the right way to do it.
Hmm...
Can you post the code where you set your validator?