Perchance those heads are...aluminum ? Very subject to corrosion, if so. Would also guess that the block is also aluminum - given the very low torque values for the head bolts.
Aluminum stinks as a metal for internal combustion - especially if ANY steel, iron, cast iron parts are used anyplace on the engine = even steel head bolts and steel piston rings. No matter what the auto makers try - overcoming the 'cathode/anode' problems of dissimilar metals, especially under high heat and temperature- can NOT be done ... not in any affordable and long lasting way.
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...