AHEM, Metalbenders don’t have magnetic powers, they Earthbend the impurities in the metal. Hiroshi’s platinum mechs were made of purified metal, so there was nothing for the Metalbenders to bend.

HOWEVER, there is someone who has magnetic powers:

Wonder how long it will be before Varrick pulls a Starscream on Kuvira…

platinum is not magnetic. let me clarify this myth; if you have a a piece of platinum that does have magnetic properties then that means it was mixed with iron, cobalt, or nickel. if his "pure platinum" mechs are magnetic then they are not pure