The spell, flre aura, seems to bug out sometimes.

-If you cast fire aura, make a new attack and use it, the added fire damage won't apply

-Fire aura randomly applies itself to mobs. I nearly got chunked out by a kobold archer because his shot did 20 extra fire damage

Are you 100% sure you made the Fire Aura spell type "User", and didn't just cast it on the kobold by mistake? I can't count how many times I've given my enemies some beneficial effects by forgetting to switch that. It would explain both behaviours at once. You can see who has the spell applied when looking at the kobold (either hold ctrl or press L and move over kim). The creature having fira aura will have an icon showing that they are affected by the spell.