Reproduction steps:
1. Create a new Universal 3D template project
2. Create > Shader Graph > From Template…
3. Select “Fullscreen Basic - URP” from Create New Shader Graph Asset window and press “Create” button
4. Create > Material
5. Select newly created Material and open Shader selection menu by clicking on the Shader dropdown in the Inspector
6. Select Newly created Shader Graph (e.g. Shader Graph > New Shader Graph)
7. Compare Material icons and preview with Shader Graph preview

Actual result: Grey rectangle is rendered at the bottom left of the Material icons and preview while no grey rectangle is visible in the Shader Graph Preview
Expected result: Material icons and preview is rendered the same as Shader Graph preview

Reproducible with: 6000.3.8f1, 6000.4.0b8, 6000.5.0a6
Couldn’t reproduce with: 6000.0.67f1 (No Fullscreen Basic - URP Shader Graph)

Reproduced on: Windows 11, macOS Sequoia 26.2 (M4)