"Multikill with any explosive" does not work/register

Had the same issue, Mukti kill with new guided rocker launcher, blown up vehicles with multiple people, double grenade kills, HEAT rounds from tank not counting. 

Only way I was able to complete this challenge was camping a hallway between the tunnel on the new map as Sundance and just spam the scatter grenade and regular grenade until competed. 

It requires 3+ kills from a single grenade. A double kill is not enough. 

..They Do NOT make that very clear...