Automatically Add Caption Shortcode to Image with Caption Field Filled In

Did a bit more digging and it appears to have been a conflict in my theme:

add_filter( 'post_thumbnail_html', 'remove_width_attribute', 10 );
add_filter( 'image_send_to_editor', 'remove_width_attribute', 10 );
function remove_width_attribute( $html ) {
  $html = preg_replace( '/(width|height)="\d*"\s/', "", $html );
  return $html;
}