Issues with temporal transition smoothing in fog shader.

I'm trying to make a relatively simple depth fog shader that chooses a fog color based on the average color of the screen (This is accomplished with mipmaps) But any time I try to incorporate data from the previous frame (in the texture "prevFrame") with itself it returns as black. Note that I can use the data from the previous frame unmodified just fine, but when I modify it at all it changes to a value of float3(0); I've tried pulling the data from a separate texture than the rendertarget, but this caused the texture to show as black regardless of modification.

#include "Tools.fxh"
    
    
    
texture2D texDepth : DEPTH;
texture2D screenAVG {Width = BUFFER_WIDTH / 4; Height = BUFFER_HEIGHT / 4; MipLevels = 9; };
texture2D prevFrame {Width = 1; Height = 1;};


sampler2D prevMerge{Texture = prevFrame; };        
sampler2D screenData {Texture = screenAVG; };


uniform float Fog_Strength <
    ui_type = "slider";    
    ui_min = 0.0;
    ui_max = 1.0;
    ui_tooltip = "Fog + Depth Buffer Blend.";
> = 0.5;

uniform float Fog_Temporal <
    ui_type = "slider";
    ui_min = 0.0;
    ui_max = 1.0;
    ui_tooltip = "Fog Color Frame Persistence.";
> = 0.9;

uniform float Frametime < source = "frametime";>;        
    
#include "ReShade.fxh"


//Duplicates the Backbuffer to a separate texture for downsampling
float3 Screen(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target
{
    float3 depth = tex2D(ReShade::DepthBuffer, texcoord).r;
    float3 blur = depth * tex2D(ReShade::BackBuffer, texcoord).rgb;
    
    return blur;    
}

//Downsamples the current frame to calculate a single value and blends it with the previous frame
float3 Temporal(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target
{
    float3 frameColor = tex2Dlod(screenData, float4(texcoord, 8, ).rgb;
    float3 avgColor = tex2D(prevMerge, 1).rgb;
    float3 temp = frameColor * (1.0 - Fog_Temporal) + avgColor * Fog_Temporal;
    
    return temp;
}


//Unrefined fog things
float3 ZN_Fog_FXmain(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target
{
    float3 frameColor = tex2Dlod(screenData, float4(texcoord, 8, ).rgb;
    float3 avgColor = tex2D(prevMerge, 1).rgb;
    float3 input = tex2D(ReShade::BackBuffer, texcoord).rgb;
    float depth = tex2D(ReShade::DepthBuffer, texcoord).r;
    depth = pow(depth, 20.0);
    float3 fog = depth * avgColor;
    input = lerp(input, lerp(input, avgColor, depth), Fog_Strength);

    return input;
}
    

technique Zentient_Fog
{
    pass
    {
        //Duplicates the buffer
        VertexShader = PostProcessVS;
        PixelShader = Screen;
        RenderTarget = screenAVG;
    }
    pass
    {
        //Modifies the 
        VertexShader = PostProcessVS;
        PixelShader = Temporal;
        RenderTarget = prevFrame;
    }
    pass
    {
        VertexShader = PostProcessVS;
        PixelShader = ZN_Fog_FXmain;
    }
    
    
    
}
 

One cannot read and write to the same texture in a graphics pass (which your "Temporal" pass does with the "prevFrame" texture).
You'll either have to double buffer the texture (add an additional pass that copies the data to a second texture which can then be read from) or use blending pass states to modify existing data instead of reading it in the shader (though looking and what the "Temporal" pixel shader does, I don't think it can be achieved with blending).