Experiment with a lower reactive mask value for stuff drawn into the reactive stencil buffer.

com.unity.render-pipelines.high-definition/Runtime/PostProcessing/Shaders/DLSSBiasColorMask.shader

@@ -37,8 +37,10 @@ Shader "Hidden/HDRP/DLSSBiasColorMask"
 
     void DLSSBiasColorMaskPS(Varyings input, out float4 outColor : SV_Target0)
     {
+        const float value = 0.7f;
+        
         UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);
-        outColor = float4(1,1,1,1);
+        outColor = float4(value,value,value,value);
     }
 
     ENDHLSL