I just checked Aegis achievements and can confirm. It's hopefully a bug. I checked a few other mechs and they don't have this problem.

I've seen a couple too, would enquire about it if it's intentional in the coming week.

I just spotted this issue in a bunch of other mech achievements. I really hope it's a bug. If not, it would be crazy. Here folks have been complaining about some achievements being unrealistic and in response they've become more unrealistic :-).

FYI I tried Stalker with longarm 12 instead of longarm 8 and the achievement does register. So looks like a bug, and luckily only a visual bug.. at least for this specific combo.