I put a commander hero with all skills in reducing unrest in a level 3 city of mine with 170 population.  net result showed that with low taxes and other buildings to ease unrest, unrest went to zero.

next turn...

city is now at zero population.

the only thing I can figure is that there is a bug that occurs when you get to zero unrest in a city?

it's definitely because of the hero being in there; if I reload to a previous save, and don't put the hero in the city, it doesn't happen.

none of my other cities show this behavior, though I have seen a couple of posts on steam of people having similar problems with their city pops randomly going to zero.

the city does not regrow the population.  basically, I'm stuck with a fully upgraded level 3 town.. with zero population.

very very expensive to raze it and rebuild, very time consuming to replay the turns from the last save game where I hadn't put the hero in there.