Seems to only affect the REV-46. The attachment not appearing here when I inspect the gun is the Force Stabilzer Handstop but it's not just that specific attachment. I swapped it to a Laser and then the Laser didn't appear in game.
Appears even though you will equip 5 in the menus loadout screen in game you will only actually get 4 attachments. For what its worth I have prestiged this gun twice, I dont know if that affects this bug.