wrong spark plugs (heat range) the pictures show over heated ceramic insulator, over torqued to engine head. Over heated & bonded with head threads.
read this for understanding
choosing the right heat range is crucial for engines. If the heat range is too cold, the spark plug will be unable to properly self-clean by burning off carbon deposits. If it the heat range is too hot, your engine could experience detonation, pre-ignition, or power loss.