REST API show update: predecessor does not get updated

If I do a PUT request to the api/v1/shows/4/ endpoint I can send a show object with a new predecessor_id, but while I get a 200 OK reply, the replied show object still has the predecessor_id set to the old value.

See this screenshot of a plain API request (done with Postman):

Screenshot_from_2018-05-18_13-01-45