There seems to be a bug with this shader in 4.5.43. When I mask it with a simple shape, it doesn't respond. It works through a surface layer, whether with coverage or not.
Uncheck 'fit mask to this'.
Is that checked by default? I never use it. Yep it is.
What exactly even if "Fit mask to this"? Why would it make a mask which already fits, gone, and why wouldn't it actually fit the mask.... to that...
The exact idea of this never dawned upon me, but it does have advantages I think. Long time ago, I remember having a situation where it worked as needed. But I can't remember what and how...