Punture’s Penumbral Ambush rune is supposed to add 15 mana cost to the skill, taking it from 0 to 15.
The Coated Blades passive is supposed to add 5 mana cost per point if it has a cost already.
For some reason, Penumbral ambush seems to be adding 5 mana cost per projectile rather than flat 15, resulting in coated blades adding 15 mana cost per point rather than 5.
If this is intended it’s not clear (and it’s imbalanced).