Assault Expert 3 not tracking grenade damage properly

The Final Assault Challenge, Assault Expert/Specialist 3, has criteria to deal 1,000 damage with grenades.  This does not appear to properly register all damage, and only a subset of grenade damage - possibly direct hit damage - is registered toward the challenge.  Multiple games as support, with multiple frag grenade kills, and my challenge registered damage is only at 33/1000.