Thanks for the maskable comment. Missed to add that. If you look at the ui source then you will see the actual stencil value will be calculated based on the number of maskable components in the parent if maskable = true and forced to 0 otherwise (see the call to MaskUtilities.GetStencilDepth()). Turning it off makes it work but you also loose the ability to stack multiple of them.
I’ll add the original response here as unity answers might not be around forever.
If you wonder about the meaning of the stencil values (3 and 6) here are the enums which they are coming from:
Stencil Compare
Stencil Operation