Why is "Simple move to location" not working as expected?

It’s quite possible your controller is not an AIController. As far as I remember TopDown’s implementation details it’s using SimpleMoveToLocation because it’s a PlayerController that’s driving the pawn on the map, and as such doesn’t have access to AI functionality. The proper solution here is to have an AIController-driven pawn, and has a body-less player controller that tells AIController where to go.