Sometimes slugs get stuck on child posts

When editing German language content in the FSD site I came across this situation:

A parent and child post of `post_type` `principle_de_de` which have the same `post_name` and which wouldn't allow the `post_name` to be edited by changing the slug below the title in the admin edit UI. I changed the `post_name` in the DB, flushed the object cache, and it now seems fine.

One to keep an eye on.

Start by reproducing the German-language FSD case with parent and child posts of post_type principle_de_de sharing a post_name. Inspect the admin slug field behavior and the object-cache state before and after editing; done means identifying why the slug becomes uneditable and confirming a reliable fix that prevents the duplicate-slug condition.

Written by the indexing model from the issue text.