That seems rather extreme. You're saying you didn't send the funds from a mixer to the exchange, just the fact that a mixer had sent funds at one point to your account was enough to get your banned? Was it the same funds from the mixer? Have you ever sent funds directly from a mixer to an exchange? This seems like a dangerous problem for the community. Essentially anyone that pays anyone using a mixer could be screwing over the person they send funds to and having their exchange accounts closed if this is true. Pretty messed up behavior.
This will also take away a person's freedom to send funds, before they even realize they are already banned because the person sending funds was detected to be connected to the mixer. This is indeed a rather extreme regulation, even though no criminal action has been committed. when someone receives payment for a signature campaign that might be an indication as well. So if you find an exchange that is too strict about the source of funds and even the funds are only a little, it's better to forget it and use another Cex, there are still many Cexes that are safe and not too strict with regulations regarding mixer addresses.