The "Rays of Precision" artifact perk on the Queensfoil Censer does not proc when defeating harpies. It seems like the harpies' corpse despawns before the ignition has time to fully activate, as the ignition animation plays, but does not complete fully. I believe I've also noticed this happening with the vex fanatics, but exploding enemies have always been weird with on-death effects. Harpies, though, are certainly an outlier. It's possible there are other enemies with short death animations that have this problem, but the harpy is the only major one I've noticed.