The challenge only requires a successful attack, not a kill. Try using a rifle. Chances are, the stealth detection is bugging out and they are detecting you at the last second. A sneak attack bonus message should appear in the corner of your screen.

I managed to stack up several sneak attacks with melee by EM blasting a squad, and then going around crouch-stabbing them.

If none of these work, then I’m not sure. Hopefully the unofficial community patch fixes it, or gets updated soon.