I have heard of this . When I first started using metallics , I noticed the bottom of my iron had "stuff" burnt on it. Since I had not starched the fabric, it was the metallic that was coming off... not enough that the metalic design was lost. But it was a mystery for awhile.
I make a good amount of quilted clothing ( to sell) that is primarliy metalics, and have learned to iron on the wrong side. Not all will have an issue , but you never know which one till you find out the hard way. Many of the fabrics that have a "glitter" is particularliy sensative to direct iron heat.