CSGO Model Black Shadow Line Between Feet

Howdy, so I’ve been using Half Life Model Viewer to tweak some hitboxes/attachments on an old player model.
I noticed that there is a black line between the players feet:

I’ve spent 3 hours now toying with a bunch of things and haven’t found an actual way to fix this.
Can somebody with CSGO modelling experience please explain the issue to me?