Glsl 2D Billboard Cutouts using Core Shaders

I've been trying to adapt Dartcats pre-1.20+ shader pack that converted some block models into 2D billboards. However instead encountered this distortion that I'm unable to resolve.
    if (abs(fract(Position.x) - 0.5) <= 0.01) {
        pos.xy += normalize(vec3(0, 1, 0) * mat3(ModelViewMat)).xy *
              (float(gl_VertexID % 4 >= 2) - 0.5) *
              (float(gl_VertexID % 8 >= 4) - 0.5) *
              2.0;
    } else {
        pos.yz += normalize(vec3(0, 1, 0) * mat3(ModelViewMat)).yz *
              (float(gl_VertexID % 4 >= 2) - 0.5) *
              (float(gl_VertexID % 8 >= 4) - 0.5) *
              2.0;
    }```