Search: managed property mapping not following "first non-blank" documented behaviour

Question

Sharepoint 2013, October 2015 CU 4763

We have an odd issue where the documented managed property (MP) behaviour of "first non-blank" crawled property isn't being followed.

Working, but for some reason requires both properties

MP : KmsTitle
CP mapping 
  Basic:display
  ows_Title (a document library with a title column)

Blank #1 (probably reasonable to be blank as basic:display is blank)

MP : KmsTitle
CP mapping 
  Basic:display

Blank #2 (not reasonable: ows_Title has a value)

MP : KmsTitle
CP mapping 
  ows_Title

The documented behaviour here is that the first "non-blank" crawled property (CP) is used as the value of the MP. In this case just a "ows_Title" should work. It does not.

There seems to be an interdependency between Basic:display and ows_Title to indicate that both of these crawled properties must be present, even though it's likely that basic:display is blank in our example.

This looks, tastes and smells like a bug or some customer-specific issue, however I'll also be happy if someone is able to provide some rationale behind this behaviour.

One of our issues is that if we use this as per the working example above then potentially a future bugfix will break our definition.