Question Terrain Tools Issue - 4th layer is black after applying splatmap

I'm having an issue applying splatmaps to terrains created using the Terrain Tools package in a URP project.

The issue is as follows...

If I have 4 or more layers set up in the Layer Palette for a given terrain and I apply a new (or existing) splatmap to the terrain, the 4th layer ends up completely black after the splatmap is applied to the terrain.

To reproduce the issue:

• Create a new terrain
• Add four layers to the layer palette
• Paint the four layers onto the terrain
• Select the terrain
• In the Terrain Toolbox > Terrain Utilities > Terrain Splatmaps, Import from Selected Terrain
• Reset Splatmaps
• Apply to Terrain

After applying the splatmap, the 4th layer is black.

The issue only seems to be present when working in a URP project. I have tested this in HDRP and the splatmaps are applied correctly.

Is there a way to apply splatmaps correctly in URP?

I am using Terrain Tools 4.0.3 and Unity 2021.2.17f1.