Is the double-tap bugged on the EVA-8?

I've noticed that when you attach the Double Tap on the EVA-8 it has a tendency to cancel out any effect that your Shotgun Bolt has on the weapon.

Once the double tap is equipped I don't believe that the shotgun bolt actually increases fire-rate at all, and when you put it back into full-auto the shotgun acts like there is no bolt attached. 

I have been avoiding the double-tap for this reason as the gun actually fires all 8 bolts faster when you use it purely with just a lvl 3 shotgun bolt.

I have no idea if this is a bug or simply a design choice?