So logging out does nothing for me but I did find a fix myself
Step 1. Buy a stack of water and divide it into singles
Step 2. Open the item recovery window in another tab
Step 3. Back to wow, vendor one water and log out
Step 4. Tab into recovery window and hit refresh(should see at least 1 new item)
REPEAT UNTIL YOU SEE YOUR ITEM YOU ARE MISSING
I think its just refreshing your item page any time you vendor, but it works, temp fix until devs can fix.