I'm curious as to its max population. Was it 2.5B or much lower?
Perhaps. I've had planets like what Feud had myself. When the PQ is upgraded, the game shows the additional tiles, but the game doesn't reconize that the planets PQ has been improved. Its annoying, because the planet doesn't increase the point my population should grow to.
Unfortunately the AI doesn't seem to have figured this out, but I trust that'll get pinned down in an update.
Maybe the AI has decided that its a too big investment to build up those planets. About 2/3rds of the AI are non-neutral in alignment, so they wouldn't benefit from the immediate terraforming that neutral gets. Come to think of it, I find that the AI is often slow when it comes to researching "Xeno Ethics" so its unlikely that the few AIs that are neutral would seek out class 1s early in the game.
Sadly, the AI is still horrible when it comes to building up their worlds. They build their most advanced structures without considering how long it would take to build up the planets. So even if they were to seek out the class 1s, it would take forever to build the industrial sectors (assuming that they built them first), let alone how long it would take to build up every thing else.
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Since we are on the subject about planets, has anyone noticed that if a terraforming tile is the last item built, that you don't get informed when finished? Normally, any other structure last built will result in a green icon to the right of the screen indicating that you finished building that planet.