Mayor/Governor automatically changing the tiles worked in my city

I am very picky about which tiles I work. Occasionally the computer will move a citizen from a tile I selected, to another tile with more production but less food.I keep every city set to default focus. How do I prevent this from happening?

Click on the tile you want to work and lock it down.

The best thing to do if you're willing to go with a little micromanagement is to set your city to production focus and manually lock down the food tiles you want to work. Food is factored in before hammers, so if you go up one population that pop will be allocated to a mine and add production to whatever you're building without costing you any food. You can then switch it to the food-high tile you want to work.

Frankly, you should lock down all tiles. Locking just some tiles will often result in the governor shifting other, unlocked citizens to tiles you don't want them to work.

I've found the production lock strategy to be kind of risky, especially if you forget to check on your cities a lot the way I do. Still, locking down all the tiles you want to work is the way to go.