EDIT: I figured it out.
The unemployed person also creates one production.
Does anyone experience this?
I have installed the following two mods for testing:
Civ V Unofficial Patch v2.5.1 (GK) (v 1)
G&K Enhanced Mod (v1.13.4) (v 1)
Apart from these two I have another mod because it sounds nice:
Reforestation (v 3)
I started a new game. From the city screen, trying to work a tile that gives one production (like a forest on a grassland) results in 0 production. Working a tile with two productions (like a hill) results in 1 production.

Does anyone experience this?
I have installed the following two mods for testing:
Civ V Unofficial Patch v2.5.1 (GK) (v 1)
G&K Enhanced Mod (v1.13.4) (v 1)
Apart from these two I have another mod because it sounds nice:
Reforestation (v 3)
I started a new game. From the city screen, trying to work a tile that gives one production (like a forest on a grassland) results in 0 production. Working a tile with two productions (like a hill) results in 1 production.