wp_get_attachment_image with custom size not rendering possible 2x srcset image

It appears the problem was that image size variants were being capped by the max_srcset_image_width property that can be raised with the following:

add_filter( 'max_srcset_image_width', 'setSrcsetMaximumWidth', 10, 2 );

function setSrcsetMaximumWidth( $max_srcset_image_width, $sizes_array ) {
  return 3200;
}

I was never aware of this before and have no idea why I never ran into this.