Flickering bug (Unity 2018.3.0.f2 + PPS)

Hi,

I've managed to create repro for an issue I found using current PPS and Unity 2018.3.0f2:

Spoiler: Shader

Code (CSharp):

1. Shader "Hidden/Custom/Grayscale"
2. {
3.     HLSLINCLUDE
4.  
5. #include "Packages/com.unity.postprocessing/PostProcessing/Shaders/StdLib.hlsl"
6.  
7.     TEXTURE2D_SAMPLER2D(_MainTex, sampler_MainTex);
8.     float _Blend;
9.  
10.  
11.  
12.     struct VaryingsExtra
13.     {
14.         float4 vertex : SV_POSITION;
15.         float2 texcoord : TEXCOORD0;
16.         float2 texcoordStereo : TEXCOORD1;
17.         float2 texcoordStereo2 : TEXCOORD2;
18.     };
19.  
20.     VaryingsExtra Vert(AttributesDefault v)
21.     {
22.         VaryingsExtra o;
23.         o.vertex = float4(v.vertex.xy, 0.0, 1.0);
24.         o.texcoord = TransformTriangleVertexToUV(v.vertex.xy);
25.  
26. #if UNITY_UV_STARTS_AT_TOP
27.         o.texcoord = o.texcoord * float2(1.0, -1.0) + float2(0.0, 1.0);
28. #endif
29.  
30.         o.texcoordStereo = TransformStereoScreenSpaceTex(o.texcoord, 1.0);
31.         o.texcoordStereo2 = o.texcoordStereo;
32.  
33.         return o;
34.     }
35.  
36.  
37.  
38.  
39.     float4 Frag(VaryingsExtra i) : SV_Target
40.     {
41.         float4 color = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, i.texcoordStereo2);
42.         float luminance = dot(color.rgb, float3(0.2126729, 0.7151522, 0.0721750));
43.         color.rgb = lerp(color.rgb, luminance.xxx, _Blend.xxx);
44.         return color;
45.     }
46.  
47.         ENDHLSL
48.  
49.         SubShader
50.     {
51.         Cull Off ZWrite Off ZTest Always
52.  
53.             Pass
54.         {
55.             HLSLPROGRAM
56.  
57.                 #pragma vertex Vert
58.                 #pragma fragment Frag
59.  
60.             ENDHLSL
61.         }
62.     }
63. }

Unity QA has been able to reproduce it and they have passed it to the developers (case 1115278). They think it could be a compile optimization issue.

I'd appreciate any insight on this issue. Thanks.