WMO creation in the modding is still pretty young and has a lot of quirks and issues like this. You might have better luck attaching lamps to M2s and placing them in the WMO as part of a doodad set or directly spawning them in Noggit. It’s more work but the upside to this method is that light will precisely radiate from your lamp/torch/etc model, potentially giving a more realistic effect than manually painted vertex lighting.