My shader stops working if my texture colours are all low-alpha

I'm using a shader with "Queue" = "Overlay" to 'paint' Texture2Ds to a RenderTexture, but I'm encountering an issue, possibly related to this (though I'm not sure).

I start with a procedurally generated Texture2D, 64x64 red pixels with preferably 0 alpha. I then blit this starting texture into a RenderTexture that I create for the sprite, and assign that RenderTexture to a SpriteRenderer sprite using MaterialPropertyBlock. I then repeatedly 'paint' sprites to this.

This all works great if I make my starting texture's red pixels have an alpha value above a certain value, and the painted sprites show perfectly, but if I go below that alpha value, the painted sprites never show up. I experimented with the alpha value by gradually reducing and found that a value of a=0.00199f works fine while a=0.00195f does not.

I thought there might be an alpha cutoff value that causes the resultant pixels to be discarded, and since I'm using the Overlay queue, the pixels may be discarded before I write to them.

Here's my shader code. There are probably some things in here that aren't needed, but I've yet to confirm this by reading up/playing around.

 Shader "Custom/DecalPaintingShader"
 {
     Properties
     {
         _MainTex ("Base (RGB), Alpha (A)", 2D) = "white" {}
     }
 
     SubShader
     {
         Tags
         {
             "Queue" = "Overlay"
             //"IgnoreProjector" = "True"
             //"RenderType" = "Transparent"
         }
         
         Pass
         {
             Lighting Off
             Cull Off
             ZTest Always
             ZWrite Off
             Fog { Mode Off }
             Blend SrcAlpha OneMinusSrcAlpha
             
              CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             sampler2D _MainTex;
 
             struct appdata_t
             {
                 float4 vertex : POSITION;
                 float2 texcoord : TEXCOORD0; 
                 fixed4 color : COLOR;
             };
 
             struct v2f
             {
                 float4 vertex : POSITION;
                 float2 texcoord : TEXCOORD0;
                 fixed4 color : COLOR;
             };
 
             v2f vert (appdata_t v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.texcoord = v.texcoord;
                 o.color = v.color;
                 return o;
             }
 
             half4 frag (v2f IN) : COLOR 
             {
                   return tex2D(_MainTex, IN.texcoord);
             }
             ENDCG
             
         }
         
     }
 } 

Appreciate any advice.

Thanks