Sometimes I want to grow a city to a massive size, for increased Science for instance. I occasionally am able to reach a food surplus of +28 per turn on one city. However, when I want to, say, change a citizen working on a Mine to a Farm, then it works (the Farm is locked down), BUT, another citizen on a Farm tile gets unlocked, and is moved back to the Mine tile when I exit the city screen. Is this because there is a maximum of +30 food/turn on a city, or something like that? Or is it just a bug?