Shader - Writing to Depth issues

I am trying to write to the depth buffer using a shader but I am facing some issues. If you look at the two screenshots, you can see that the mask is a circular gradient (black part) but it does a awful cut-out and I don't understand why.

It's quite strange because the gradient seems to be good if I render the Depth buffer from the camera. But inside the shader it does a bad simple cut-out instead of a nice gradient.

How can I fix this ?

 Tags { "Queue"="Geometry+10" }
 ZWrite On
 Blend SrcAlpha OneMinusSrcAlpha
 Pass
 {
     CGPROGRAM
     #pragma exclude_renderers gles flash
     #pragma vertex vert
     #pragma fragment frag
     #include "UnityCG.cginc"
     sampler2D    _Mask;
     float4        _Mask_ST;
     
     struct VertOut {
         float4 pos :    POSITION;
         float2 uv :    TEXCOORD0;
     };
     struct FragOut {
         float4 color :    COLOR;
         float depth :    DEPTH;
     };
     VertOut vert(appdata_base IN) {
         VertOut o;
         o.pos    = mul(UNITY_MATRIX_MVP, IN.vertex);
         o.uv    = TRANSFORM_TEX(IN.texcoord, _Mask);
         return o;
     }
     FragOut frag(VertOut IN) {
         FragOut o;
         float4 mask    = tex2D(_Mask, IN.uv);
         o.depth        = mask.rgb;
         o.color.rgb    = mask.rgb;
         o.color.a    = 1.0;
         return o;
     }
     ENDCG
 }