3D objects counting as a mask for sprites

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I'm using URP with a 3DRenderer, when I add a sprite & sprite mask with no background, it works just as expected where the sprite is correctly masked by the sprite mask. However, when there is any 3D object behind it (e.g. a quad, using a URP/Lit shader), it's counting as a mask so the mask I want stops working.

I'm guessing the 3d objects are messing with the stencil buffer but I'm not sure how to debug/investigate it. Any help would be much appreciated!

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Check if the Universal Renderer asset has any renderer features (e.g. RenderObjects) overriding stencil values for those objects. And/or the global stencil override at the top

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thanks - I already tried disabling all renderer features, and there's no stencil override set.