aksl

On a further thought, you could just strip the condition together and always append the original size to the output, because the browser will decide the image via the srcset attribute. The only thing that should be added is the watermarking of original image. This would be the code for the relevant part, with the condition being commented out:

//if (\count($mapped_sizes) === 0) {
  $width =
    \hacklib_cast_as_boolean(\array_key_exists("width", $imagedata))
      ? ((int) $imagedata[\hacklib_id("width")])
      : null;
  $height =
    \hacklib_cast_as_boolean(\array_key_exists("height", $imagedata))
      ? ((int) $imagedata[\hacklib_id("height")])
      : null;
  if (($width !== null) && ($height !== null)) {
    $url = \wp_get_attachment_url($id);
    // added: watermarking of the original image
    if (\hacklib_cast_as_boolean($watermark)) {
      $url = self::get_watermark_url($url, $query_args);
    }
    $url = Utils::get_protocol_relative_url($url);
    $mapped_sizes[] = array(
      "url" => $url,
      "width" => $width,
      "height" => $height,
      "name" => "original"
    );
  }
//}

Might cause some side effects(?), not sure if the ‘sizes’ array element of the get_image() response is used in other cases.

Gravity Forms form with a file upload field used for image uploads.

I was able to reproduce the issue while not logged in. Disabling the firewall rule “file_upload Malicious File Upload (Patterns)” fixed the issue, so it seems to be the root cause. If it helps, the PHP version in use before 7.2 was 7.0.

Workaround for the issue (translates slug to ID without applying the wpml_object_id filter):

add_filter( 'text_blocks_show_text_block_id', 'fix_tb_wpml_bug' );

function fix_tb_wpml_bug( $id )
{
    if ( !is_numeric( $id ) ) {
        $page = get_page_by_path( $id, null, 'text-blocks' );
        if ( $page ) {
            $id = $page->ID;
        } else {
            $id = 0;
        }
    }

    return $id;
}