How to specify custom UV's for casted shadow with a custom shader?

Questions
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1

Hi, I’m currently rendering a plane with a 2d image as a sprite, using a custom shader that offsets the uv’s for the texture based on the frame I’m trying to render.

    vec2 atlasUv = (vUv * uvScale) + vUvOffset;
    vAtlasUv = vec2(atlasUv.x, atlasUv.y);


    vec4 inputColor = sRGBToLinear(texture2D(
      colorMap,
      vAtlasUv
    ));

I’m successfully able to set the UV for the texture, and the alphaMap, by using some of the shader chunks that set the corresponding uniforms/variables. I’ve confirmed that the alphaMap is accurate as well, by setting the successfully outlines the visible portion of the texture.

    // This updates the UV's for the alphamap
    vAlphaMapUv = vAtlasUv; 


    // I've tested that this works - 
    vec4 inputColor = sRGBToLinear(texture2D(
      atlasMap,
      vAtlasUv
    ));

The shadow that is being cast onto other objects still seems wrong, though. The weird thing is, it seems to be casting a shadow in the shape of the bottom left frame, no matter which frame I try to show.

How does the shadow’s shape get calculated? Is there a way to modify the UV’s to match that of e.g, the alphamap?