If they pre-generated the txid they shouldn't forward it to the user but actually leave the withdrawal status at pending or something.
I have never seen a service that I use forward me a txid that hasn't been broadcasted to the network. Makes little sense in my book. It only causes unnecessary confusion.
I've never actually experienced it but I've seen a few similar cases here, like this one for example
Coinbase pending transaction - No transaction found with the hashDefinitely agreeing with the confusion part but there's also the possibility that it was not intentional and was just a result of their faulty system. In any case, it doesn't make the situation any better.