After spending 6 hours trying to figure this out, I realized I can't set the destination by dragging the Nav Mesh Agent into the State. Instead, I used the Set Agent Destination action from the Path Finding package and it worked.
What is super confusing about this obvious fix is how it worked in the editor-- which is why it didn't occur to me that it's literally not even part of PlayMaker yet without the additional actions.
6 hours... wtf...
At least it works now...