This isn't because of magnetism: it's water in contact with straight iron and stainless steel. (Saved you a click - the owners identify this).

A classic stainless steel failure mode is if it gets contaminated by iron particles (i.e. regular steel) during processing (you have to use not Fe grinding discs, keep posts separate etc).

The magnet trapped water, and the particularly low grade stainless used in the truck did the rest.

Yet another entry in this is literally the stupidest vehicle on the road though.