There are some ingredients within flavors that interact with plastic, whether it be packaging or equipment. This is particularly true for ingredients that are commonly used in Cherry and Amaretto flavorings. When flavored coffee beans are left in the grinder for a number of days, and having that constant interaction with the flavored (potentially slightly wet) beans can cause pitting in the plastic. This especially happens when the oils mix with the chemical in the low-quality plastic of some grinder brands.
The best thing to do would be to not leave the beans sitting in the grinder. If the hopper was made of glass, it would not be an issue.
All of our flavorings are certified GRAS and are recognized as safe for consumption by the FDA.