I thought I'd bring some attention to this since this one definitely seems like a bug. When you melee kill an enemy with Winter's Guile, you gain 1 stack of Warlord's Sigil, which increases your melee damage by a large amount and can stack up to 5x. Each time you gain a stack, the buff duration becomes around 4 seconds and refreshes for additional melee kills. When you melee kill a Vex Fanatic without the buff, you do not get the buff. But when you melee kill a Vex Fanatic [i]with[/i] the buff, you gain a stack, but the duration does not refresh. So it's an issue with refreshing the duration from the looks of it. Haven't tried it with other exploding enemies, but it probably does the same thing.