Why don't soft particles work when using render objects feature?

I'm making a portal effect using VFX Graph, default particle shader, and soft particles turned on in the graph. I'm using render objects features in URP asset renderer to create a stencil effect in the plane where the portal sits. The soft particles work fine when colliding with that plane, until I change the plane to the layer which is masked in the URP asset renderer. When the plane is rendered using the render objects feature instead, the particles don't soften when they intersect with it.

I've tried experimenting with the render objects feature for the plane's layer, and none of the stencil settings or depth settings solve the problem. It seems like the soft particles don't work on any layer which is masked in the URP asset renderer. I'm guessing this has something to do with the rendering order or deferred lighting when I mask a layer in there, but I don't know enough about how that works. Is there a way to have soft particles work in this case? I'm using URP and Unity 2021.1.11.

Thanks for any advice!