Spirit of the Necrotic causes Warlock melee ability to bug and not be counted as a melee/ability kill. This prevents it from activating things like Devour (requiring an ability kill) and Facet of Bravery (requiring a powered melee kill). This issue can easily be recreated by equipping a Solipsism with Spirit of the Necrotic with the Devour aspect and then getting a kill with the powered melee. If you do not have Spirit of the Necrotic then Devour procs on powered melee kill but when you equip the Solipsism with SotN your powered melee will no longer be counted as an ability kill and will not proc Devour. My best guess is that SotN is treating the whole melee attack as poison damage instead of the initial powered melee then applying a poison DoT.