Well, it seems you can take advantage of the AI with this before they can.

I saw an AI fleet heading to a Harpy controlled star and sent a colony ship to it timed to arrive on the same turn.

When it got there I was able to establish a colony, and then the Harpies and Humans fought it out. The ...

So I've noticed that the AI is sometimes able to send a fleet to star controlled by Star Harpies, lose the battle, yet still manage to colonize a planet in the star.

Is there a trick to doing that, is it a bug, or just an AI thing?

OK, I think I might see what it is. If they are able to escape ...