The oven, shelves and trays may begin the second batch in a different thermal state. A hot reused tray can affect food before the timed cooking stage even starts.
What the observation means
That difference is separate from a thermostat fault. The recipe may expect a cool tray or another preparation condition. Neighbouring cavity use can add further context.
A useful next step
Follow the recipe and appliance guidance for successive batches. Keep tray temperature and preheating history with any report of changing results. Do not alter internal controls or apply water to a hot tray or cavity to force it back to the first batch's conditions.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.