Economy Production Quests

This isn't exactly a bug, but it is overly challenging.  The Economy production quests seem to be net rather than gross.   For example, if you have resources going to science, then that lumber doesn't count.  Well, for stone and lumber, I can reset the research to zero to get them to count.   The real problem is the food.  It will not count the food going towards population and army.   I don't have an overly large population yet, but every time I upgrade the resource sources (forest, garden, etc), it adds yet more population.  Trying to get enough food to register for 250 is requiring quite high gardens, and I'm only on my first few days of my new town.  I fear I'll hit a point where I'll never be able to complete the quest.

Don't worry. The quest is not impossible to complete. The population growth will pleatue at a certain point (unless you expand your military forces). Then you'll be able to increase your resource production (even beyond triple-digit with appropriate strategic resources).
The game itself tries to balance your progress in resource production as well. During the early days of the game, food improves lumber, lumber improves stone and stone improves food production. This circle of resource development will make it easier to upgrade all resources at a similar pace.

One more tip to get extra food is to move the army (including scouts/spies) temporarily to herald/opposition this way they no longer consume food in town.

Or, if you are in a hurry use the hoe (Hors' hoe of course, shame on you for thinking of something else) for a 25% boost to gross food income (that is, applied before consumption).