This might be caused by a fix that supposed to make the AI scout faster and know if a planet is unoccupied earlier, so they can work with less waste in defences. Who knows. But I would consider lucky, especially if absurd AI, a freebie is always welcome over the hard time the AI usually likes to give.
This happened to me again tonight. It was a 2-planet system. I was on one planet, and the AI was on the other planet. The AI did not expand at all. I sent a massive army to the second planet expecting some kind of defense, but the enemy commander was there all by himself. Very disappointing. I backed this game for the single player experience, and so far the release version has been a major disappointment.
I just played a game and the AI basically built 4 things then did nothing. Then when i attacked it, it moved to the next resource for mining and tried to built around there, built the same things then stood and did nothing. This was us both being on the same planet.
