Shader behaves differently in every API

#include "ReShade.fxh"

#define PIXELATE_MIPLEVELS 8
#define PIXELATE_MIPLEVELS_UNIFORM_MAX 7

uniform int iUIMiplevel<
    ui_type = "drag";
    ui_label = "LOD";
    ui_min = 0; ui_max = PIXELATE_MIPLEVELS_UNIFORM_MAX;
    ui_step = 1;
> = 0;

texture2D texMipmap { Width = BUFFER_WIDTH; Height = BUFFER_HEIGHT; MipLevels = PIXELATE_MIPLEVELS; Format = RGBA8; };
sampler2D samplerMipmap { Texture = texMipmap; };

float3 MipMapPS(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target {
    return tex2D(ReShade::BackBuffer, texcoord).rgb;
}

float3 PixelatePS(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target {
    return tex2Dfetch(samplerMipmap, int4((int2)floor(vpos.xy / (iUIMiplevel + 1)), 0, iUIMiplevel)).rgb;
}

technique Pixelate {
    pass {
        VertexShader = PostProcessVS;
        PixelShader = MipMapPS;
        RenderTarget0 = texMipmap;
    }
    pass {
        VertexShader = PostProcessVS;
        PixelShader = PixelatePS;
        /* RenderTarget = BackBuffer */
    }
}