A radiant zone can cycle as its control regulates heat. The glass and pan retain energy even when the visible glow disappears.
What the observation means
Looking at the element does not measure the food's temperature. A cooking result that has changed substantially still needs its own description.
A useful next step
Follow normal cooking guidance and keep hands clear. Report whether the zone never heats, cycles differently or fails to switch off. Do not defeat a limiter or raise the setting continually just to maintain a constant red glow beneath the glass.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.