It's a quirk of the physics engine used in the game, I could see it taking a massive effort if the solution was to move to a different engine and would take a long time to implement, never happening.
It's not like it isn't fun.

Maybe a simple workaround would be to change the force vector direction when an actor (because it happens with every character, it's not a funny death feature just for the player! You can see sometimes bandits or your followers shoot in the sky by the giant hits) gets hit by the giant mace, and/or the gian mace blow itself.
The idea would be to change the vector from vertical/perpendicular to the ground to a small slope, maybe this would workaround the issue.