Transmute and possible error

[Sortvind]

I am not gonna shout out; Zomg a bug!!!
Rather, i am gonna ask you if you can clear something up for me.

The Ability Transmute, magic section, it gives you mana after killing a fiend and x2 that amount if one of your troops gets killed. Easy enough.
What i don`t get, is why i don`t get the mana if you kill the fiend using magic?
Might not be a bug, but it is damn annoying when i kill 3 using fireball and i could be in + instead of - 20 mana during that cast.
When i write kill fiend, i mean the whole stack.

All help and reply is appreciated