I've also read enough about this but it has never been an issue for me because regulators are on the necks of exchanges as well so they have every right to know their customers more by asking for the source of their funds. Regardless of whether your funds look genuine or not, if it is big to an extent and they couldn't do the internal transfer verification to ascertain whether the funds have been small in value before they grew to that big level due to investment purposes, they might ask you to prove your source of finds.

This is an international practice to counter money laundry. Also, if your crypto address has been linked to bad transaction(s) initially, it is possible for them to ask for additional proof of that fund to exonerate yourself. As for me, I don't see this as a big deal, if your money is clean, just give them what they want and both parties will be good.