Mag Plug gasket

Yes, hard plastic that definitely compressed and started to crack with the full 18 ft lbs of torque.    The only thing I can think of is that during initial testing,  there is procedure that requires hooking a pressure transducer to this port, and wondering if the hard plastic gasket was used for this setup and then mistakenly the gasket was transferred to the magnetic plug.   

Clearly not a good material for any sort of gasket... luckily it did not cause you any problems while on there.