[Snake]

Impure Element's Void Energy-to-Null Damage drawback activates upon chanting a spell (1st use), and again when releasing the spell (2nd use).

It should not work on cast (1st use of any invocation), since it does not use any Elemental ATK yet as you are preparing it, I think. And sometimes that same damage works to break the invocation, which makes this very janky.

[Neus]

This is not a bug, just how Impure Element works.