Normaly if you hold the right click zen use 5 ball and your ammo decrease 5 percent (15/20) when you ammo is lower than 5 zen use how much ball you have e.g if you have 3 ball in your ammo normaly (overwatch 1) stock 3 ball around and push it. But now ı play zen if i have a 1 ball in my ammo(1/20) ı can fire 5 ball.
Its video : Overwatch Zenyatta ammo bug. - YouTube