Edits not being saved for custom fields on relationships (duplicate of CRM-20382)

I'm duplicating issue CRM-20382 here, because that issue hasn't been touched in almost two months, remains unassigned, and is tagged as "trivial" when in fact it should be tagged as "critical".  I don't know how to change the priority on that issue, so I'm opening a new one.

To be clear: for two versions now, it is impossible for users of CiviCRM to edit custom fields that they created on relationships.  If person A is promoted at organization B, it is not possible to edit the "role" field and indicate that the promotion occurred.  Worst of all, there is no indication that the update failed, so our clients believe that they are updating their data when in fact their edits are being silently discarded.

This is a critical issue for many of our clients; I'm about to investigate the possibility of downgrading their Civi installs, because as it stands now the sites are too badly broken to use.