The concealment kills are a slog. Any time your stealth meter is orange and people are looking for you, your sneak attack isn't available. It's best to go on a planet that has lots of little non hostile grubs, and sneak attack them. Otherwise you're sitting in a base waiting to go full hidden and the enemies to stop searching for you. Good luck getting those kills took me several hours to get the appropriate kills.

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.