Drupal 8 menu item hierarchy displays as root level if parent is external

If you create a hierarchy under a menu item, change that menu item's url to an external link, and clear the cache, it will flatten the hierarchy underneath the parent with the external link and put its children in the root of the menu. The parent link relationship will be maintained, so you can change it back to an internal link, clear the cache, and have it go back to normal.

To reproduce, create a menu item with however many children, then change its url to an external link, and then clear the cache. The issue will be apparent wherever the menu appears and on the menu management page.