Furnace doesn’t channel with Smelter’s Wrath. You hold down to channel Smelter’s Wrath and there’s no Furnace. Broken node.
No, it definitely works, but there is definitely a visual bug occurring. A single blast of flame will come up when you first start channeling and nothing afterwards, but watching health bars enemies are still taking damage while in the AoE. I assume Furnace’s AoE is the same as Smelter’s Wrath but with the missing graphics it’s hard to know for sure