Had this issue for the first time last week, when I was doing Varik's bounty "Get kills with arc abilities". Went into Lv 41 arena with my titan. Was using pulse grenades. The problem only occurs with Taken enemies, not with the normal encounters. Especially bad in the Vex arena, but also with the others. When I threw a grenade to the area, where some enemies were just spawning, it hit the ground, did one pulse and simply disappeared, therefore killing nothing. The same happened, when I threw it after the black fog, from which the enemies spawn, had already been gone. Yesterday, it was even worse. Went in with my solar titan and thermite grenades. Again in the Fallen vex arena, threw the grenade towards some enemies, it hit the ground and nothing happened at all, it instantly disappeared not sending out even one flame burst... Witnessed the same with hunters arc bolts as well. Hitting the ground, not connecting to anything, simply gone. Xbox One. Please have a look.