Shader not working correctly on iOS after 2018->2020 upgrade

After upgrading to Unity 2020.22.3f1 some of our shaders have stopped working on iOS builds. Most notably some colors will tend to white and some degree of transparency is lost.

This is an example of a shader that used to work fine for basic shadow decals and now turns whitish:

Code (CSharp):

1. Shader "Custom/Shadow Decal" {
2.     Properties
3.     {
4.         _MainTex ("Texture", 2D) = "white" {}
5.         _Fade ("Fade", Float) =  1.0
6.     }
7.     SubShader
8.     {
9.         Tags { "RenderType"="Transparent" }
10.         ZTest Always
11.         ZWrite Off
12.         LOD 100
13.        
14.         ZTest Always
15.         ZWrite Off
16.  
17.         Blend SrcAlpha OneMinusSrcAlpha
18.        
19.         Pass
20.         {
21.             CGPROGRAM
22.             #pragma vertex vert
23.             #pragma fragment frag
24.             // make fog work
25.             #pragma multi_compile_fog
26.            
27.             #include "UnityCG.cginc"
28.  
29.             struct appdata
30.             {
31.                 float4 vertex : POSITION;
32.                 float2 uv : TEXCOORD0;
33.             };
34.  
35.             struct v2f
36.             {
37.                 float2 uv : TEXCOORD0;
38.                 UNITY_FOG_COORDS(1)
39.                 float4 vertex : SV_POSITION;
40.  
41.             };
42.  
43.             sampler2D _MainTex;
44.             float4 _MainTex_ST;
45.             fixed _Fade;
46.  
47.            
48.             v2f vert (appdata v)
49.             {
50.                 v2f o;
51.                 o.vertex = UnityObjectToClipPos(v.vertex);
52.                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
53.                 UNITY_TRANSFER_FOG(o,o.vertex);
54.                 return o;
55.             }
56.            
57.             fixed4 frag (v2f i) : SV_Target
58.             {
59.                 // sample the texture
60.                 fixed4 col = tex2D(_MainTex, i.uv);
61.  
62.                 col.a = (1.0 - col.r) * _Fade;
63.                 col.rgb = fixed3(0.0, 0.0, 0.0);
64.                
65.                 UNITY_APPLY_FOG(i.fogCoord, col);
66.                
67.                 return col;
68.             }
69.             ENDCG
70.         }
71.     }
72. }
73.  

Any ideas what this could depend upon?

 

Seems to be this same issue: https://forum.unity.com/threads/reg...1-ios-with-post-processing-unchecked.1196239/

 

Try to put your device in Dark Mode and see if it helps.

 

Hi, thank you @xyome. Strangely that actually works but in the end, since we were pressed for time and could not downgrade anymore, we ended up resorting to a replacement of the fragment shaders that wouldn't work with some basic surface shaders that did roughly the same things. That's how we "solved" the issue, at least for now.