BUG: When you've got opened Marketplace and you buy something from buy orders you don't get visible tab (tab is not appearing) with "Completed auctions". To see and collect bought items you need to close marketplace and open it up again. What else, you even don't get updated number of "Completed auction" if the tab exists when you've got opened Marketplace. To see exactly how many are they, you need to click on that tab and then number of auctions updates.
when I buy something from the market and it get's sent to the Completed Tab, it shows this tab for me.
Please check if, when you buy something, you have the "Transfer completed purchases directly to inventory" enabled or not. Otherwise, if possible, capture the bug so we can see what's going on there.