Why does masked animation look so strange?

I have an idle and a shooting animation.
Both animations are perfectly tailored to my model.

The idle animation looks like this:

And the shooting animation looks like this:

I have 2 animation layers:

The "Base" layer plays the idle animation.
The "Pistol Shooting" layers plays the pistol shooting animation, and it has an avatar mask because it should only influence the upper body.

The results however look like this:

What might be wrong here? What could I check?