316 stainless is "barely" not magnetic to begin with and only if it is properly annealed. Even if it is a good quality 316, it can start to become magnetic after being cold worked...i.e. put under a lot of repetitive force without heat....which it certainly has been in this case. How they were welded may have also affected the magnetism of the material unless they were annealed after welding. It is possible that these bolts are 316 even though a magnet is attracted to it after the life they have been through.