A cover changes how directly the surface receives heat and how moisture escapes. A paler or softer top can therefore follow the cooking method rather than an oven failure.
What the observation means
Covering a dish is different from lining the cavity or wrapping an entire shelf. The latter can obstruct heat flow and may be prohibited by the appliance instructions. A loose foil edge can also move towards hot parts. Follow the recipe's directions for a securely placed cover and any point at which it should be removed.
A useful next step
Record whether the dish was covered when comparing results. If its surface stayed pale but the contents cooked, describe those outcomes separately. Do not use extra foil as an improvised repair for uneven heating, and keep every accessory clear of vents and heating components as the manual requires.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.