I started using the "Mantle of Fire" spell judiciously late game. The +1 melee damage/per level was hard to pass up. It took me a bit to realize there was an issue, but I realized the Champions I was casting the spell on were getting very powerful...too powerful to be accounted for by leveling and gear improvements.

Looking at the "Attack" value for the units in question (Outside combat), it looked too high. Mousing over the number shows me  "Fire Attack   +49."

Bottom line, I was able to figure out the Mantle of Fire buff to damage does not go away after combat. Repeated castings over several combats will cause the spell to stack on the previous bonus and increase over time.

I confirmed this using two different Champions and several comparisons before and after combat where I cast the Mantle of Fire spell.

(Edit: Confirmed this to be in 1.01 as well)