After reading the twitter post about the TWAB preview (the one about gunsmith mats), I hopped on the game and started spending them. After receiving a message that my postmaster is full, I deleted some of the items and started spending the mats again. After spending some more materials, the warning message doesn't pop-up again so I thought all is still ok. Eventually, I checked the postmaster and ended up seeing that some precious items (ascendant shards/prisms and raid spoils) are all gone. This annoyed me a lot and the fact that the warning message popped up again after a third cycle of spending mats made me even more frustrated. Hard earned items (that can't be vaulted) get pushed out of the postmaster, but a random weapon (that has a low chance of being a god roll) ends up in the postmaster. I personally thought that the weapon gets deleted, but apparently not so i guess this is a warning message for others to not do the same thing as me.