The mica plate burnt because of oil, grease or residues splashed on it for a long time and carbonized after heating. After heating fish, meat and other food, the oil and proteins splashed because of the high temperature and stuck to the mica plate. They are too close to the heat source which makes them easy to burn. Therefore, after a long period, the mica plate is scorched or burnt.
Please replace the mica plate with a new one, and clean up the food residues and grease after each use.
Note:
If the mica plate is damaged, food residues will go through the cavity into the magnetron, which causes the magnetron damage. So if using the microwave oven to heat food frequently, please pay attention to the internal cleaning and maintenance of the microwave oven.