Ok, I did some testing and it's the way I'm exporting the .vrm somehow... I grabbed the model you linked, brought it into blender and exported it and it's not showing any of the slider options in the Avatar tab. Is there a specific way I need to export it for the slider to work?
by default the model already has sliders, so re-exporting isn't needed unless you want new/different ones. if it worked beforehand but not after you changed it then there's something you're missing
