There have been multiple such cases. Not only because of AML high risk positive after an transaction from some address that is black-listed, but it can be for any reason the exchange wants it to be. They can make up literally any reason and call it suspicious. Using a VPN? Suspicious. Having too much money? Suspicious.
If they want to, they can freeze your funds and nobody should be naive enough to think it won't happen to them.
Remember: not your keys, not your coins. Do not store funds on a third party wallet which someone else has access to. You will regret it.