The issue is the keyword declaration in the CustomLighting.hlsl file.
It's set up as two different keywords that are always enabled:
#pragma multi_compile _MAIN_LIGHT_SHADOWS
#pragma multi_compile _MAIN_LIGHT_SHADOWS_CASCADE
But in the way URP works is that shadows can be disabled or one of the shadow modes can be enabled. The issue is fixed by replacing those two lines with:
#pragma multi_compile _ _MAIN_LIGHT_SHADOWS _MAIN_LIGHT_SHADOWS_CASCADE