'Clay Pigeon Shooting' Achievement not progressing

I still haven't found a solution for this odd situation yet, but for some reason killing snipers with flechette rounds (on any shotgun I use) doesn't lower the counter of how many I need to kill for the 'Clay Pigeon Shooting' achievemtn.

For example, I was in a Nightclub mission where I shot 3 snipers, checked my inventory, and the progress was still at 0/10.

The mission is bugged and will not properly display how many snipers you've killed. I've done this mission before and you just have to keep a mental check of how many snipers you've killed. Once you've killed 10, you'll get the achievement and the relevant weapon mods.