Custom field data on relationships only saved when creating a new relationship, not on edit

I have replicated this on the CiviCRM demo website.

Create a custom field for a relationship type. Create a new relationship between two contacts and put some data in the custom field. It saves correctly. It does not save if you try to edit it. Likewise, adding new custom data to an existing relationship does not save: it only saves when you create a new relationship.

No errors are given.