This problem is most commonly called splitting or silver splitting. The hot stamping foil is very uncommon in lighter shades, including golds, but does occur occasionally with darker metallic colors. The reason that splitting sometimes occurs is because the lacquer coat, which contains the colorant that produces the desired shade, doesn't bond correctly to the aluminum layer of the foil. First, if the adhesive coat (sizing) is not consistently applied, certain areas of the foil roll may not release from the carrier properly, causing the lacquer/color coat to stay on the carrier while allowing the adhesive and metal coat to transfer. As a result, silver spots can appear on the foil stamped image where the lacquer did not transfer. The second reason splitting can occur is when there is too tight of a release between the lacquer and the polyester carrier. When there is a strong bond between the lacquer and the polyester and a good bond between the sizing (adhesive coat) and the substrate, you can get a splitting between the lacquer and metalization. This is caused by a lack of release agents when the release coat is applied to the foil roll.
One way to test the foil for potential splitting problems is to take a piece of scotch tape and secure it down on the adhesive side of the foil. Slowly pull the tape up and check to see that there is a clean transfer between the lacquer and the polyester carrier. If you still see some of the color coat remaining on the carrier, you may have a splitting problem with the foil on press.