My cities will frequently use a handful of unemployed citizens even when there's other, better choices available. In a recent game (in a prior GEM version), my sizable capital had nearly a dozen unemployed citizens, but didn't fill up all the engineer slots 
When this occurs, the only thing to do is manually select every single tile. Leaving even one tile untouched and removing the unemployed will cause the AI to de-employ the remaining tile(s) to "fill" the vacancy
Selecting a Hammer focus seems to exacerbate the issue; likely because it places additional weight on hammers, but its not exclusive. I've seen several unemployed citizens with a Balanced focus.

When this occurs, the only thing to do is manually select every single tile. Leaving even one tile untouched and removing the unemployed will cause the AI to de-employ the remaining tile(s) to "fill" the vacancy

Selecting a Hammer focus seems to exacerbate the issue; likely because it places additional weight on hammers, but its not exclusive. I've seen several unemployed citizens with a Balanced focus.