Damage bug with Ogryn Fist Grenade

I was playing around in the Meatgrinder and noticed the Ogryn’s Fist weapon does less damage when you do the Special attack which is a Charged Punch with a Grenade to the face.

I’ve taken screenshots of the various attacks to demonstrate what I found.
Top Left - Charged Punch - 190 Damage
Top Right - Aimed Grenade launcher - 511 Damage
Bottom Left - Special Punch with Grenade - 400 Damage
Bottom Right - Charged Punch with separate Grenade attack

As you can see, just using the Grenade launcher dealt 511 damage while the Punch+GL did only 400, so this makes using that attack completely pointless as it does less damage.