Object cache breaks the Post Relationship Editor on single (related) posts

Symptoms

When you use object cache mechanisms such as Redis or APCU Object Cache, then on posts in a post relationship, the related posts in the Post Relationship Editor are not displayed.

When you connect or create new connected posts, these will get connected properly, but will not appear in the post relationship editors of single posts.

Workaround

There is no solution for this but to remove object cache for the time being.

Note that the posts are still related properly, so a View listing related posts, still will output the expected results.