Custom Fields: Contact Reference Fields do not update on Contact merge

A custom contact reference field does not update to merged contacts result.

Example: Custom Group Humour Extends CivicRM Contact. It contains a Contact Reference Field 'Mentor'. Suppose Tim Hawkins is the Core Contact and for his Mentor is Bill Simmons. Suppose there are 2 different(yet to be merged) Bill Simmons Contacts in CiviCRM: Billy Simmons, and Bill Simmons. Someone has set Bill Simmons(cid=10) as Tim Hawkins Mentor. Someone else merges Bill Simmons into Billy Simmons, deleting Bill Simmons(cid=10) and leaving Billy SImmons(cid=4). The custom field 'Mentor' with cid=10 for Tim Hawkins will not be updated to cid=4.