Underbarrel Mount Attachment Starts with Reduced Ammo

The extra ammo provided for the M32A01 HE is inconsistent when using the Assault class with different gadget/weapon combinations (see loadout scenarios below). 

results: M32A01 HE starts with 1 ammo
scenario: 
- primary weapon, with underslung mount
- gadget 1 with additional primary weapon
- gadget 2 with M32A01 HE

results: M32A01 HE starts with 2 ammo
scenario: 
- primary weapon, NO underslung mount
- gadget 1 with additional primary weapon
- gadget 2 with M32A01 HE 

results: M32A01 HE starts with 2 ammo
scenario: 
- primary weapon, with underslung mount
- gadget 1 with anything-but-additional-primary-weapon
- gadget 2 with M32A01 HE

results: M32A01 HE starts with 2 ammo
scenario: 
- primary weapon, with underslung mount
- gadget 1 with M32A01 HE
- gadget 2 with anything-else

results: M32A01 HE starts with 2 ammo
scenario: 
- primary weapon, NO underslung mount
- gadget 1 with M32A01 HE 
- gadget 2 with anything-else

What is causing M32A01 HE to start with one or two ammo? Is it to balance having a second primary weapon? Is it to balance the accessibility of my "Underslung Mount" ? 
Or is this a bug?