Weird "PIP" bug in Direct3D 10 with repro

#include "ReShade.fxh"

// ----------------------------------------------------------------------------
// TEXTURES 

sampler2D BackBufferSampler {
    Texture = ReShade::BackBufferTex;
};

/* For keeping current frame in quarter size */
texture2D Downscaled {
    Width = BUFFER_WIDTH / 4; 
    Height = BUFFER_HEIGHT / 4; 
    Format = R8;
};
sampler2D DownscaledSampler {
    Texture = Downscaled;
};

/* For keeping previous frame in quarter size */
texture2D PreviousFrame {
    Width = BUFFER_WIDTH / 4; 
    Height = BUFFER_HEIGHT / 4; 
    Format = R8;
};
sampler2D PreviousFrameSampler {
    Texture = PreviousFrame;
};

// ----------------------------------------------------------------------------
// FUNCTIONS 

float GetSmallBackBuffer(
    float4 vpos : SV_Position,
    float2 texcoord : TEXCOORD) : SV_Target {

    float3 color = tex2D(BackBufferSampler, texcoord);
    return (
          color.r * 0.25 
        + color.g * 0.50 
        + color.b * 0.25 
    ); 
}

float SaveSmallBackBuffer(
    float4 vpos : SV_Position,
    float2 texcoord : TEXCOORD) : SV_Target {

    return tex2D(DownscaledSampler, texcoord).r;
}

float3 ShowDifferences(
    float4 vpos : SV_Position,
    float2 texcoord : TEXCOORD) : SV_Target {

    float3 color = tex2D(BackBufferSampler, texcoord);
    
    /* Highlight what changed between current and previous frame */
    
    float smallCurrent = tex2D(DownscaledSampler, texcoord).r;
    float smallPrevious = tex2D(PreviousFrameSampler, texcoord).r;
    
    return (color * 0.5 + 0.5) + (smallCurrent - smallPrevious);
}

// ----------------------------------------------------------------------------
// PASSES

technique Repro {
    pass Downscale {
        VertexShader = PostProcessVS;
        PixelShader = GetSmallBackBuffer;
        RenderTarget = Downscaled;
    }

    pass Differences {
        VertexShader = PostProcessVS;
        PixelShader = ShowDifferences;
    }

    pass SaveFrame {
        VertexShader = PostProcessVS;
        PixelShader = SaveSmallBackBuffer;
        RenderTarget = PreviousFrame;
    }
}

float3 DoNothingShader(
  float4 vpos : SV_Position, 
  float2 texcoord : TEXCOORD) : SV_Target {
  
  return tex2D(ReShade::BackBuffer, texcoord);
}

technique Repro {
  ...
  
  pass DoNothing {
    VertexShader = PostProcessVS;
    PixelShader = DoNothingShader;
  }
}