Bogged is a new mob in 1.21 that is a skeleton with 2 mushrooms on its head. If you shear it, it drops these. The PlayerShearEntityEvent should fire but does not (yet). There was a similar issue with Snowmen a while back. 

Code to check (just a logger on the registered event):

public class ShearingListener extends Listener {
    @EventHandler(priority = EventPriority.NORMAL)
    public void onShear(final PlayerShearEntityEvent e) {
        Bukkit.getLogger().info(e.getEventName());
    }
}