The AI settled a 0/0/0 tile, which is in itself a bug. Look at the bottom left of that screenshot, where it shows how much production/food/essence your city has. See how your city's got all those zeroes? That's a bug, and I've noticed it in FE as well.
Can't believe I didn't notice this, and I do know that that tile wasn't 0/0/0, there was good tile there, as I was going to settle there myself, apparently it settled on the 1 tile that wasn't fertile.
