When a dropper is powered to place an item inside a hopper, the item is not recognized by a comparator placed next to the hopper. Given the same conditions the comparator DOES recognize an item placed into chests, trap chests, droppers, and dispensers so I assume this is a hopper glitch.

The comparator does turn on when an item is placed inside or removed from the hopper, if the comparator is toggled, or if the comparator is removed and replaced.