Mists of Deception -- exploit??

[Edi]

K said:
I don't think this is a bug, but a feature.

At that level of play, if all spells dropped when the caster left then you'd rarely see a use for battlefield enchantments like this. This level of play is so deadly that most battlefield spells rarely are as effective as just having one mage casting blasting spells, so nerfing them further would be to nerf them out of play.

It just so happens that we have Kristoffer on record being surprised that battlefield enchantments did not end when the caster left the battle and saying this should not happen, so it's a confirmed major bug.