Are you going to eat more food with UBI? Probably not.

Are food prices going to rise if others eat more food? No, we have spare capacity for food production.

Also, a bigger topic, but supply and demand is not linear like they teach in school. Food prices are fairly cheap and stable in America because of excess capacity plus the fact that producers face thin margins plus spoilage. It will take a lot before prices move "commensurately".

This is just one example.