Why does Avatar Mask not work as expected?

I have 2 layers: The base layer, and an upper body layer. On the upper body layer, I have an avatar mask because I only want to affect the upper body. However, this mask doesn’t work as expected.

Here is what it looks like:

enter image description here

This is what is looks like when the upper body layer has a weight of 0:

enter image description here

And this is with the upper body layer with a weight of 1 and with the Avatar Mask:

enter image description here

As one can see, the lower body is affected. The avatar mask doesn’t seem to have an effect. Is that a bug in Unity, or is there anything I could still check?

Thank you!