Unusual mana cost with Frostbolt and CoC Cold Snap

Cast Frostbolt with scattershot support and hit an enemy while having Cold Snap on Cast on Crit setup and triggered would instant drain my mana down to bottom, same with Frostbolt hit Frostwall with same setup when enmey around.
My guess is when Cold Snap explode the Frostbolt, it counts as using Cold Snap but with Frostbolt mana cost.
Please fix this ASAP, thank you.