Membership custom fields don't get updated in secondary member's membership record when primary membership is edited

While this does not affect a major functionality, there is no UI workaround.

Steps to reproduce:
Create a relationship type
Add it to a membership type
Create a new custom field, applied to membership
Add the relationship to two (or more) contacts
Add a membership to one of the contacts - enter the custom field
Check the other contact's membership record >>> the custom field has propagated and is accurate

Edit the primary member's membership - adjust the custom field
Check the secondary member's membership record >>> the custom field is the same as before the primary membership record edit

Discovered on CiviCRM 4.6.9 on Joomla 3.4.4; reproduced on the demo site w/ CiviCRM 4.6.0