Battlefield spell + retreating?

[Twan]

It would make it hard to protect fragile mages throughout the battle which you need to cast the spells initially with. Then your mages have to be equally powerful or actually even better than the Thugs that you're facing. If somebody blesses your troops, then you don't expect that effect to vanish when the priest leaves or even is killed, too. But I could actually live with the decision that as soon as a mage leaves the battlefield, his spells vanish.

I think he was speaking about battle enchantments only, not all spells.

And for battle enchantments it's a known bug.

Now, out of extreme exploits like combos with MoD, it may be considered in the same category as a reverse communion, a non expected feature everyone can use and not bad for the game (note that communion slaves able to cast is also something listed in the short list).