I have learned and seen that the worst coffee beans are used for instant coffee. Worst coffee beans means "very low grade coffee (in case of Guatemala, Prime or Extra prime style coffee)" and " broken and floating coffee beans" they do not roast well since density and size are different than other normal good beans.
and when you talk about flavored instant coffee, that is the worst of the worst because they want even want to cover up the bad taste of inferior coffee with flavors.
Same for the Flavored roasted beans. Normally they use inferior beans to cover up the real coffee taste. Please stay away.