Actually, that is the occupied city symbol, not the revolt symbol. You should turn on Citizen Management to verify what tiles they are working, but it looks like they are working the 2-food desert farm tile, the 3-food/1-gold oasis tile and the 3 gold luxury tile (desert wine?), with nary a hammer among them.
You should mouse hover over the food item to see how that is calculated here, but I can't make this foot to what appears on screen. From what is visible, you should be generating 9 food (2 from granary plus 3 from oasis plus 2 from desert farm plus 2 from city center tile). 9 food minus 6 food for 3 citizens, leaves 3 surplus food, minus the 75% unhappiness discount, gets you to roughly to 0.75 food, not 0.4 food. And that calculation leaves out any food multipliers from your empire (e.g., +15% from Tradition finisher and such).
The city center should be providing 2 food and 1 hammer, and it apparently is not. Did you get this city in a peace treaty? There was a bug that caused cities received in peace treaties to get no yield from their city center, but I thought that was fixed. You might post this in the Bug Reports sub-forum of General Discussions.