Hi @X-perto,
Choose this visualization option to make sure the custom stencil is working in your project. If this view appears empty, then something is wrong on your meshes or your project settings. If this is appearing correctly, then the problem is indeed the materal.
That being said, if you are having issues getting that method to work within the material, this is the more common method for testing against a stencil value:
Good luck! Let me know if you have any other quesitons.