Hey there. The usual cause is that when you first complete Leviathan, the shotgun you get is not the 'complete' version, it is the 'quest' version. After obtaining this, there were additional steps to finish it, including killing Cabal and obtaining Emperor Seals from more Leviathan encounters. Doing this would upgrade it to its true form, adding a second shot to the magazine and unlocking its weapon perks. This is also where the weapon becomes unlocked in collections and on the Red War: Exotics collection badge. Sounds like you did not complete those steps and therefore still had the quest version of the weapon. As you did not complete it and the quest has now been vaulted, the only thing you could have done was to dismantle, and you'd need to purchase it from the memorial kiosk at the Tower.