The 8 direction movement is designed for control by human input, and it's not designed for automatic control, which is why (I guess) nobody's tried to make pong like this before. It's really easy to control automatically with some simple events - or an automatic behavior like Bullet - but 8 direction is the wrong choice.
What was the exact problem? You didn't specify. I'm assuming it's the object moving sideways. If you disable rotation in the behavior to stop it trying to set the object angle, and always set its Y coordinate to a certain value, it seems to work OK, but as I said, best find another way to do it.