Sub-grid + missing for many to many relationship

We have a many to many relationship between account and contact. Sub-grid for contacts with multiple accounts is on the account form. 

There is no plus "+" and grid view buttons on sub-grid command bar whereas they are available for the other sub-grid with 1:N relationship on the same form. 

There is "ADD EXISTING CONTACT" button available if going to the contact associated view for the relationship. 

I tried removing few display rules for the Mscrm.AddExistingRecordFromSubGridAssociated command for Contact on the button and it makes no difference.

I understand this is an out of box behavior (link attached below) but still wonder if there is any way to customise it so user could have the the same experience for 1 to many and many to many relationship sub-grids on the same form. 

http://community.dynamics.com/crm/b/gonzaloruiz/archive/2013/04/29/availability-of-add-new-and-add-existing-buttons-in-sub-grids

It would great if there is any work-around for this.

Thanks,
Jacqui