Maybe I'm just confused, so please correct me if so, but it seems like the DoT that the tooltip says is applied to enemies within 5m, isn't. Enemies within 5m do take the immediate damage they should and Writ of Fire is spread. It's just Combustion's own DoT that doesn't seem to be applying.
Combustion has never had it's own damage over time component, probably getting confused by a tooltip error (a lot of skills have been getting weird merged tooltips lately).
Ah, you're right, the final section of the Combustion tooltip is showing the exact effects of Writ of Fire. But it shows that whether WoF is on the target or not. That's quite misleading and makes the skill look more powerful than it is.