You can extinguish a campfire with a water bucket and reignite it with a flame arrow.

I know that WorldGuard doesn't currently catch that, and the Spigot BlockIgniteEvent.IgniteCause does not reflect that possibility. I've pored over the Spigot sources and I can see where this is implemented in n.m.s.BlockCampfire. I haven't been able to find any Spigot API hooks in the obvious suspects (CraftCampFire, CraftArrow).

I do admit, I haven't raised a bug with the WorldGuard guys about this yet, mainly because their dev server has experienced hardware failure. But I'm reasonably confident that the lack of a Spigot API for flame arrows igniting things indicates that it has not yet been implemented by the Spigot project.