Laser Designate targets that sustain damage

I've been trying to complete some challenges and one challenge is bugged for me. I have been scanning tanks/light-tanks/vehicles with the dart and the laser scanner. I got 1.092/2.000 but after that i don't seem to get any more points. It looks like that counting is bugged. I tested it with a friend of mine and it doesn't work. It only seem to work with javelins (sometimes).

Please fix this challenge so i can go further with the grind, thanks!