For opaques use decal diffuse alpha for all channels.

For transparent, if material alpha is less than decal alpha replace it with decal alpha.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/DecalData.hlsl

 #if _COLORMAP
 	surfaceData.baseColor = SAMPLE_TEXTURE2D(_BaseColorMap, sampler_BaseColorMap, texCoordDS.xy);
 	surfaceData.baseColor.w *= totalBlend;
+	totalBlend = surfaceData.baseColor.w;	// base alpha affects aall other channels;
 	surfaceData.HTileMask |= DBUFFERHTILEBIT_DIFFUSE;
 #endif
 #if _NORMALMAP

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/DecalUtilities.hlsl

 	matMask |= mapMask;
 }
 
-void ApplyBlendDiffuse(inout float4 dst, inout int matMask, float2 texCoords, int sliceIndex, int mapMask, float blend, float lod)
+void ApplyBlendDiffuse(inout float4 dst, inout int matMask, float2 texCoords, int sliceIndex, int mapMask, inout float blend, float lod)
+	blend = src.w;	// diffuse texture alpha affects all other channels
 	dst.xyz = src.xyz * src.w + dst.xyz * (1.0f - src.w);
 	dst.w = dst.w * (1.0f - src.w);
 	matMask |= mapMask;
 	matMask |= mapMask;
 }
 
-void AddDecalContribution(PositionInputs posInput, inout SurfaceData surfaceData)
+void AddDecalContribution(PositionInputs posInput, inout SurfaceData surfaceData, inout float alpha)
 {
 	if(_EnableDBuffer)
 	{
 			if(diffuseIndex != -1)
 			{
 				ApplyBlendDiffuse(DBuffer0, mask, positionDS.xz, diffuseIndex, DBUFFERHTILEBIT_DIFFUSE, decalBlend, lod);
+				alpha = alpha < decalBlend ? decalBlend : alpha;	// use decal alpha if it higher than transparent alpha
 			}
 
 			if(normalIndex != -1)

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLitData.hlsl

 #endif
 
 #ifndef _DISABLE_DBUFFER
-    AddDecalContribution(posInput, surfaceData);
+    AddDecalContribution(posInput, surfaceData, alpha);
 #endif
 
 #if defined(DEBUG_DISPLAY)

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitData.hlsl

     surfaceData.tangentWS = Orthonormalize(surfaceData.tangentWS, surfaceData.normalWS);
 
 #ifndef _DISABLE_DBUFFER
-    AddDecalContribution(posInput, surfaceData);
+    AddDecalContribution(posInput, surfaceData, alpha);
 #endif
 
 #if defined(DEBUG_DISPLAY)