Inconsistent skill behaviour with WASD vs. Mouse/Keyboard on Molten Blast Skill

The skill behaves totally inconsistent dependent on how you have it set up and what input method you use:

WASD + Attack without moving checked =
Forward movement / Backward attack
Backward movement / Backward attack

WASD + Attack without moving unchecked =
Forward movement / Forward attack
Backward movement / Backward attack

Mouse + Keyboard + Attack without moving checked =
Backward movement impossible
Attack always backwards

Mouse + Keyboard + Attack without moving unchecked =
Backward movement impossible
Attack always forwards

I made a video showing the differences:

https://www.youtube.com/watch?v=hXbEnz1jizs

Reddit discussion forced me to think about this more. Some people seem to think this might be expected behaviour.

I guess what irritates me is the fact that you can only use it in the way it was likely meant to be used if you play with WASD. So that you if your character moves backwards, you also step backwards while creating and throwing/shooting/batting the projectile and that if you move forward, the whole creating/throwing/shooting/batting also creates a forward movement.

You simply cannot do that with Mouse/Keyboard. You have to choose either/or and therefore it feels wrong in 50% of the situations. If I move towards a monster, I want the skill to move forward. If I move away from a monster, I want it to also take a step backwards.