Closely stacked trays can shield food and restrict circulation between loads. The intended shelf spacing is part of the oven's cooking arrangement, not simply a matter of fitting everything inside.
What the observation means
A deep dish beneath a low shelf leaves less room for heat movement than a shallow sheet. Some multi-level programmes specify particular rail positions to preserve those paths. Filling every possible level may therefore produce different results from the manufacturer's recommended arrangement, especially with bulky cookware.
A useful next step
Check the loading diagram for the chosen mode and reduce crowding where the instructions require it. Record whether the undercooked area was hidden behind another tray. Do not remove shelf-support parts or balance cookware on improvised spacers. If suitable separated loads still show an unusual change, provide the actual shelf positions with the repair enquiry.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.