Yeah it bothers me too, tbh. The visual HUD indicator for demonic core indeed is more reminiscent of a priest or paladin themed visual than anything warlock would have.
I guess it’s supposed to represent the fire portion of the damage in demonbolt, but I would’ve hoped for a shadow/fire visual effect and not just fire. It’s a shadowfire spell after all.
But moreover, even the way that they did the visual effect with the fire theme is off if they’re set on going for that. It in no way looks like any spell themes in warlocks arsenal that I can think of.
Something more of a darker hued flame and/or chaotically designed would be more fitting imo. It’s both way too orderly, and way too brightly hued to be anything I’d associate with warlocks.