I have a custom record to track technical details for customers/contacts.  The record is setup as a child of the Customer record.  I also have a Contact field to associate the record with a specific person.  This is setup as a child of the Contact -- and is sourced by the Customer/Parent.  This way, only Contacts of the Customer appear in the selection list.  This works perfectly.

The custom records appear in both Customer and Contact tabs.  I can create a new record from this tab in the Customer record, however, if I try to create it from the Contact record the Contact field does not stick upon saving the record.  It initially autopopulates the Contact because it was created from that entity, but upon save it disappears.  I have to re-edit the record and enter the Contact again to associate it with the Contact.