'Heavy Weaponry' Achievement not unlocking on Steam

I'm missing an achievements that should have been unlocked on Steam.

The achievement;  "Heavy Weaponry" - "Get 5 kills with LMGs without reloading in Multiplayer" does not unlock on completion on Steam. 

I made 8 kills without reloading.
I was using the L110 Hatchet at that moment. That specific skin is from pre-ordering the game. Could the issue have something to do with using skins not being default and therefor not being marked as LMG or registered with Steam achievements?

See attached video with evidence of completion.

Edit: I added a screenshot of the "Unstoppable" dog tag/accolade which proves that it did in fact register my 8 LMG kills without reloading, but still didn't trigger the Steam achievement.