Referring to the ToS the casino would help in this situation. Some casinos made it clear that funds from mixers might undergo some checks or not allowed. These casinos have to abide by anti-money laundering laws and practices as it applies to the gaming industry.
There is no need to take the risk of transferring mixed funds to casinos when you have other options. Mixed funds can be cleaned before transferring them to the casino. But it will be ideal to avoid using mixers if the funds will be used for gambling. You might have to go through complicated KYC particularly if you win big.
I think there’s always an easy way to avoid that. When you’re using a mixer, don’t send it directly to the casino. Just create one address as a bridge wallet, so it looks like it’s coming from a single wallet. I’ve done that from time to time and never had any issue with casinos flagging my account.
So I think that method is effective for now, but of course, we never know what could happen in the future.