custom fields attached to relationships are unsearchable

to reproduce:

• create a custom data set used for relationships
• create a field and mark it searchable
• go to a contact with an existing relationship (or create a new relationship) and enter a value for that custom field
• visit advanced search and search that field by the value entered

expected result: only the record with the searched data is returned.
actual result: all contacts with a relationship are returned.

I was able to reproduce in both 4.6 and 4.7. so it seems this is a long-standing bug.