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:

     Shader "Custom/Shadow Decal" {
         Properties
         {
             _MainTex ("Texture", 2D) = "white" {}
             _Fade ("Fade", Float) =  1.0
         }
         SubShader
         {
             Tags { "RenderType"="Transparent" }
             ZTest Always
             ZWrite Off
             LOD 100
             
             ZTest Always
             ZWrite Off
     
             Blend SrcAlpha OneMinusSrcAlpha
             
             Pass
             {
                 CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
                 // make fog work
                 #pragma multi_compile_fog
                 
                 #include "UnityCG.cginc"
     
                 struct appdata
                 {
                     float4 vertex : POSITION;
                     float2 uv : TEXCOORD0;
                 };
     
                 struct v2f
                 {
                     float2 uv : TEXCOORD0;
                     UNITY_FOG_COORDS(1)
                     float4 vertex : SV_POSITION;
     
                 };
     
                 sampler2D _MainTex;
                 float4 _MainTex_ST;
                 fixed _Fade;
     
                 
                 v2f vert (appdata v)
                 {
                     v2f o;
                     o.vertex = UnityObjectToClipPos(v.vertex);
                     o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                     UNITY_TRANSFER_FOG(o,o.vertex);
                     return o;
                 }
                 
                 fixed4 frag (v2f i) : SV_Target
                 {
                     // sample the texture
                     fixed4 col = tex2D(_MainTex, i.uv);
     
                     col.a = (1.0 - col.r) * _Fade;
                     col.rgb = fixed3(0.0, 0.0, 0.0);
                     
                     UNITY_APPLY_FOG(i.fogCoord, col);
                     
                     return col;
                 }
                 ENDCG
             }
         }
     }