You can control what you get from faction rank up packages (Destiny)

by Chappy, Arlington, VA., Monday, June 27, 2016, 12:16 (2860 days ago) @ unoudid

I see the same thing - if the drop is for a gear slot that's already full it goes directly to the postmaster. If I rank up multiple ranks at once (thank you heavy ammo synths), the second or third package can't be redeemed until I leave the vendor menu and then return.