I had a colony on a world with ruins. My engineers uncovered robotic troops. I didn't notice that my troops were hurting, so I ordered them to attack.
The robots destroyed my troops, but my troops must have simultaneously destroyed the robots. I got a message that my colony had surrendered, along with the ships in orbit. Yet the colony was still there, I still had my ships, but my troops were gone, and I don't see any enemy. I did not get a message about the enemy units destroyed (that I recall).
I think it's a bug, but maybe its not.