Terrain becomes significantly brighter every four Terrain Layers added using the "Simple Blend 4PlusLayers URP" shader

How to reproduce:

1. Open the attached “IN-150071” project
2. Open the scene “SampleScene”
3. Expand the "4LayersPlusURP - With Terrain Sample 4 Plus Shader" GameObject in the Hierarchy
4. Add four additional Terrain Layers to the Terrain component (Inspector > Terrain > Paint Terrain > Edit Terrain Layers > Create/Add Layer)
5. Observe the Scene view

Actual result: The terrain using the Terrain Sample "Simple Blend 4PlusLayers URP" shader becomes significantly brighter for every four layers added
Expected result: The terrain brightness remains consistent regardless of the number of layers added

Reproducible with: 6000.3.22f1, 6000.5.9f1, 6000.6.0b9, 6000.7.0a5
Could not test with: 6000.0.0f1, 6000.0.82f1 (“Simple Blend 4PlusLayers URP” shader does not work)

Reproducible on: macOS 26.5.2 (by user), Windows 11
Not reproducible on: No other environments tested