When steel is heated above its Curie point (about 770 deg C for low carbon steel) it loses any magnetic field in the steel and the atoms are all scrambled so they are in random orientations. So maybe
one heat shield is getting hotter than the other and getting demagnetised. Just pure speculation (and possibly not based on the correct physics)
Dave
Blue 4/4 1969, Green +4 1953, (different) Green +8 1977
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