UPDATE: I just received the small amount from the forex broker. I guess the old address is still valid and tied to my account. Quite a nerve wracking experience though, I had a lot of money in that forex account and brokers are notoriously strict about AML policy partly because they don't want people to withdraw.
So I guess that exchange is an exception from the usual, so they send withdrawals like a non-custodial wallet.
-snip-
You mean an address previously used for my account may eventually be used for someone else?
No, I was talking about most Exchange's usual withdrawal procedure, as explained by Danny.
They'll consolidate all the users deposits to a hot wallet and use some of it for withdrawal, so the address used for withdrawal is in their control, not the user.