Geometry mask won't hide meshes

After having to re-export my mesh many many times to get substance painter to somehow recognize different mesh parts so I could work with geometry mask, for my surprise geometry mask doesn't work.

at least not how it used to, as seen in this vid: https://www.youtube.com/watch?v=TGASuIGSUns

it properly "masks" whatever I select to paint, but only while still showing the entire mesh. I want to paint my character's tongue without having to make it a separate texture set. what gives?