Mobile shader Offset different on different phones

I've got an interesting problem: I have some UI with a custom shader that uses Offset -1, -500000 to render it on top of some level geometry. On a Samsung S6 and Note 4, as well as in the editor, it looks like this:

On my S3, it looks like this:

Why? Do some mobile devices not support Offset?

-500,000 was chosen because it's the only thing that works (I need world space UI to render on top of some 3d geometry). Lower numbers only partially shift the UI where it needs to be. This is also developed for mobile VR, which is why I need world space UI in the first place.

For reference, here's the shader:

 Shader "UI/Offset" {
      Properties {
          _MainTex ("Font Texture", 2D) = "white" {}
          _Color ("Text Color", Color) = (1,1,1,1)
          
          _StencilComp ("Stencil Comparison", Float) = 8
          _Stencil ("Stencil ID", Float) = 0
          _StencilOp ("Stencil Operation", Float) = 0
          _StencilWriteMask ("Stencil Write Mask", Float) = 255
          _StencilReadMask ("Stencil Read Mask", Float) = 255
  
          _ColorMask ("Color Mask", Float) = 15
      }
  
      SubShader {
  
          Tags 
          {
              "Queue"="Transparent"
              "IgnoreProjector"="True"
              "RenderType"="Transparent"
              "PreviewType"="Plane"
          }
          
          Stencil
          {
              Ref [_Stencil]
              Comp [_StencilComp]
              Pass [_StencilOp] 
              ReadMask [_StencilReadMask]
              WriteMask [_StencilWriteMask]
          }
          
          Lighting Off 
          Cull Off 
 //         ZTest Off
          ZWrite Off
          Offset -1, -500000
          Blend SrcAlpha OneMinusSrcAlpha
          ColorMask [_ColorMask]
  
          Pass 
          {
              CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
  
              #include "UnityCG.cginc"
  
              struct appdata_t {
                  float4 vertex : POSITION;
                  fixed4 color : COLOR;
                  float2 texcoord : TEXCOORD0;
              };
  
              struct v2f {
                  float4 vertex : SV_POSITION;
                  fixed4 color : COLOR;
                  float2 texcoord : TEXCOORD0;
              };
  
              sampler2D _MainTex;
              uniform float4 _MainTex_ST;
              uniform fixed4 _Color;
              
              v2f vert (appdata_t v)
              {
                  v2f o;
                  o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                  o.color = v.color * _Color;
                  o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
  #ifdef UNITY_HALF_TEXEL_OFFSET
                  o.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
  #endif
                  return o;
              }
  
              fixed4 frag (v2f i) : SV_Target
              {
                  fixed4 col = i.color;
                  col.a *= tex2D(_MainTex, i.texcoord).a;
                  clip (col.a - 0.01);
                  return col;
              }
              ENDCG 
          }
      }
  }