Mana getting consumed when GREATER LIGHTNING BOLT is getting triggered

Mana getting consumed when GREATER LIGHTNING BOLT is getting triggered.

I have CHOIR OF THE STORM AMULET.

Is this expected behavior for triggered skills??

Because I don't see any mana cost associated for that skill (GREATER LIGHTNING BOLT)

Archmage has a flat +5% mana cost per skill - includes triggers. Same thing is happening with my Doomblast. Not sure if it's intended or not.