The settings describe different ways of delivering heat around the food. The temperature printed for one mode should not automatically be transferred to the other.
What the observation means
Fan circulation changes the movement of hot air, while conventional cooking relies more on the distribution from the selected upper and lower heating arrangements. Recipe writers may therefore give different temperatures or timings. There is no single correction that is reliable for every recipe, oven and quantity. A familiar meal cooked on a newly selected mode is not a controlled comparison with its previous result.
A useful next step
Use the recipe's matching mode and the manufacturer's shelf guidance together. Record whether the poor result began when the cooking function changed, rather than simply saying the thermostat is wrong. If food is repeatedly undercooked, do not rely on appearance alone to judge its safety. Follow appropriate food cooking instructions and discuss the appliance concern separately.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.