Workaround for an issue with local keyword and shadergraph preview material.

com.unity.render-pipelines.high-definition/Runtime/Material/TerrainLit/TerrainLit.shader

@@ -80,7 +80,7 @@ Shader "HDRP/TerrainLit"
     #pragma multi_compile_instancing
     #pragma instancing_options assumeuniformscaling nomatrices nolightprobe nolightmap
 
-	#pragma multi_compile _ _ALPHATEST_ON
+	#pragma multi_compile_local _ _ALPHATEST_ON
 
     // All our shaders use same name for entry point
     #pragma vertex Vert

com.unity.render-pipelines.high-definition/Runtime/Material/TerrainLit/TerrainLit_Basemap.shader

@@ -51,7 +51,7 @@ Shader "Hidden/HDRP/TerrainLit_Basemap"
     #pragma multi_compile_instancing
     #pragma instancing_options assumeuniformscaling nomatrices nolightprobe nolightmap
 
-	#pragma multi_compile _ _ALPHATEST_ON
+	#pragma multi_compile_local _ _ALPHATEST_ON
 
     #pragma vertex Vert
     #pragma fragment Frag

com.unity.shadergraph/Editor/Drawing/PreviewManager.cs

@@ -799,6 +799,8 @@ void BeginCompile(PreviewRenderData renderData, string shaderStr)
                     ShaderUtil.UpdateShaderAsset(shaderData.shader, shaderStr, false);
                 }
 
+                CoreUtils.Destroy(shaderData.mat);
+
                 if (shaderData.mat == null)
                 {
                     shaderData.mat = new Material(shaderData.shader) { hideFlags = HideFlags.HideAndDontSave };