A substantial cold load absorbs heat from its surroundings. The preheat signal describes the oven's readiness before loading, not a promise that conditions cannot change afterwards.
What the observation means
A heavy casserole and a light baking sheet place different demands on the cavity. Opening the door and inserting the load occur together, so a temporary change is not automatically evidence of a failing heater. The important observation is whether the appliance proceeds normally under the loading arrangement described by its instructions.
A useful next step
Follow the recipe's guidance on preheating and whether the dish itself should be cold or preheated; do not improvise with heat-sensitive cookware. Record an unusually prolonged failure to recover, together with the mode and load. Avoid repeatedly removing and replacing the dish to check the display, because that changes the conditions you are trying to describe.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.