This whole issue is troubling. I have purchased a good deal of silver and gold bullion from Apmex. If they sold you something listed as "new," it is deeply problematic that it is fake. I can give them a little bit of a break if it was sourced from the secondary market and listed as a "generic" 10 ounce bar.
A little background: I have a relative with a jewelry store that was very successful in the "cash for gold" business and have been taking gold and silver bullion to NTR for years. In addition, I sold a good deal of bullion that I owned to NTR. All in all, I have taken thousands of ounces of silver bullion in all increments to NTR. Much of it was from Apmex.
I have never seen what you encountered. I don't doubt it is possible. I once encountered a Walking Liberty half-dollar that weighed 8 grams and was fake. I also have a war nickel that is magnetic. I have seen fake Carson City silver dollars, and Ebay is littered with fakes.