Ran across something coincidentally. The arm that holds the magnet on manually adjusted brakes can, under certain circumstances, ride up on the back edge of the engaged brake shoe and then prevent the shoe from fully disengaging. One thing that can cause this is a worn or ill fitting bushing on that arm allowing excessive side play. I don't know that that is what happened here or why it should happen on new brakes. But it can be a cause of a hanging brake. And it is something to check for when doing a brake inspection.