You do not own addresses at exchanges, you borrow them from exchange so you can deposit coins to them. And then when someone else or you request withdrawal of coins exchange picks one or more such addresses and send coins from them to destination address. From my experience, it is highly unlikely that once you request withdrawal of coins that you will receive the same coins you have deposited to exchange. You will highly likely get coins from some other user or users depending on how much coins they have deposited and how much coins you are withdrawing. So for example, if you and two other users deposited 100,000 CANN in total and some other user requests withdrawal of 100,000 CANN exchange might pick all coins from your deposit address and all coins from deposit addresses of other two users. No coins will remain at addresses you and other two users used to deposit coins but that does not mean exchange stole those coins!

You own as many coins as it is written somewhere at exchange, probably at balances page. For as long as you can withdraw that many coins it is all fine.