Perhaps the problem lies in the fact that coins were made from some alloy that have slightly different properties.
I can only witness what I have seen - our coins didn't show signs of any reaction before rubbing, but after rubbing the reaction was quite interesting and visible - but didn't last long, as the mercury was taken away by the groving layer of aluminum oxide so we have never seen more than a few milimiters thick grey mass being mixture of oxide and mercury.
Perhaps additional problem was grease on the surface? We have never tried degreased coins, they were just taken from our (rather shallow
) pockets.