Porto - Menu hierarchy is not preserved when syncing menus

Overview of the issue

When using the Porto theme, you may notice that submenu levels are not preserved after syncing menus from WPML > WP Menus Sync page.

Workaround

Add the following custom XML configuration in WPML > Settings > Custom XML Configuration:

<wpml-config>
  <custom-fields>
    <custom-field action="ignore">_menu_item_type</custom-field>
    <custom-field action="ignore">_menu_item_menu_item_parent</custom-field>
    <custom-field action="ignore">_menu_item_object_id</custom-field>
    <custom-field action="ignore">_menu_item_object</custom-field>
    <custom-field action="ignore">_menu_item_target</custom-field>
    <custom-field action="ignore">_menu_item_classes</custom-field>
    <custom-field action="ignore">_menu_item_xfn</custom-field>
    <custom-field action="ignore">_menu_item_url</custom-field>
  </custom-fields>
</wpml-config>