I remember seeing some of the shittier exchanges (and gambling sites) that were doing things like this when the mempool was flooded... as they would also "spend" unconfirmed change, this would end up generating chains of like 50+ unconfirmed transactions.
There were countless "It's been X hours, transaction not confirmed! HELP!!!1!!!!!1111!!!!"-type threads that popped up in the "Technical Support" board back in December/January that featured this sort of thing.
Ugh, I remember those days. I started out trying to help like others, before realising after a while that most of them had repeatedly spent low-fee txs, and were just looking for a free ride.
In fact, @OP, I recognise your name and this isn't the first time you've experienced this... it's a real shame exchanges are still skimping on fees when they're already quite low now. Time to ditch that exchange, I'd say, no? For important txs like yours, need to use your own wallet to pay the right fees for the desired confirmation time.