You can see here that the trees always show outside the URP stencil mask here - is this a limitation of three.js stencil? Works fine in Unity viewport but not in local server/glitch through three.js.
I made one tree stump show only with 1 material and that is masked correctly
I think I ran into this issue a while back trying to get my main scene to NOT show through a stencil. I will check if the main scene was using a multi material if i can find it again