Actually, there is no bug here, simply another discrepancy.

Here's the short Transmutation info from the russian version:

Converts the death sigh to mana
Mana is doubled for friendly units
Killing a summoned unit grants 1 mana, friendly summoned unit - 2.
Mana is NOT given when a stack dies from a SPELL, only from a direct attack by a unit.

I wonder how many things that have failed to translate properly in this game.

Thank you all for clearing that up for me!

Well, you DO get mana if you drop a trap and the creature dies from the that.

I remember getting mana from units killed in trap from transmute normally, but no rage. Other direct damage spells however, generated rage normally but no mana from transmute.

On the otherhand, I think I also recall the ball lighting dragon skill generating a nice amount of rage, though DG was right in that its direct damage skill generates no rage (Though Ball lighting is the only indirect not counting summon)

I think spells and dragons skill that works indirectly are the exception to the general rules stated previously. Please correct me if I remembered wrong.

That sounds right. Also, killing things with Rage abilities will trigger transmute. It is a pretty simple system. If you spent rage, you don't get rage unless the damage is delayed. Likewise with mana for transmute.