Non static occludee hidden by occluder

Hi,

On a VR project, I had some GUI I wanted to be drawn over everything, no matter what, but since I had to use World Space canvas, the best I came up with was to add a material to all my drawn GUI element using a shader to have it drawn over everything (I don't fully understand the shader, I found it on the internet while looking for means to draw world space GUI over everything).

I've had no problems with this technique, actually, until now, it has worked wonders. But then I tried mixing that with occlusion areas and, somehow, my GUI gets hidden behind occluders.

Of course, neither my GUI nor any of it's parents, nor children are marked as occludee static, so I don't understand why it disappears. I did check that it was an occlusion problem by checking that the item was still in the scene, but when going into the "visualisation" tab of the occlusion window, it is hidden.

Any hint on what I might be doing wrong? Here is the code of the shader :

 Shader "UI/MenuOverEverything"
  {
      Properties
      {
          [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
          _Color ("Tint", 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"="Overlay+1" 
              "IgnoreProjector"="True" 
              "RenderType"="Transparent" 
              "PreviewType"="Plane"
              "CanUseSpriteAtlas"="True"
          }
           
          Stencil
          {
              Ref [_Stencil]
              Comp [_StencilComp]
              Pass [_StencilOp] 
              ReadMask [_StencilReadMask]
              WriteMask [_StencilWriteMask]
          }
   
          Cull Off
          Lighting Off
          ZWrite Off
          ZTest Off
          Blend SrcAlpha OneMinusSrcAlpha
          ColorMask [_ColorMask]
   
          Pass
          {
          CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
              #include "UnityCG.cginc"
               
              struct appdata_t
              {
                  float4 vertex   : POSITION;
                  float4 color    : COLOR;
                  float2 texcoord : TEXCOORD0;
              };
   
              struct v2f
              {
                  float4 vertex   : SV_POSITION;
                  fixed4 color    : COLOR;
                  half2 texcoord  : TEXCOORD0;
              };
               
              fixed4 _Color;
              fixed4 _TextureSampleAdd; //Added for font color support
   
              v2f vert(appdata_t IN)
              {
                  v2f OUT;
                  OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
                  OUT.texcoord = IN.texcoord;
  #ifdef UNITY_HALF_TEXEL_OFFSET
                  OUT.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
  #endif
                  OUT.color = IN.color * _Color;
                  return OUT;
              }
   
              sampler2D _MainTex;
   
              fixed4 frag(v2f IN) : SV_Target
              {
              half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;  //Added for font color support
              clip (color.a - 0.01);
              return color;
              }
          ENDCG
          }
      }
  }
 

Note : I'm currently using Unity 5.4.2f2