When activating the inner zone of a cooking field, it seems that the outer zone is also activated.
Both the inner and the outer zones appear red.
Radiant hobs with two-zone cooking fields or three-zone cooking fields
There is no malfunction. The red color on the outer ring is caused by an optical illusion. The light from the inner zone is illuminating the outer zone and it therefore appears that the outer zone is also activated.
To test this, you can activate one, two, and then three zones and carefully observe how the light and color of the zone behave.