Without commenting on whether a cashiers check is "cash" in any context, I think you will find that the record retention requirement is triggered by the purchase of a monetary instrument with "currency." A cashiers check does not fit the definition of "currency" and purchasing a monetary instrument with another monetary instrument does not generate a record retention requirement.

You will find other threads here that debate whether the activity is reportable as "suspicious," but those threads generally end with the observation that the bank could prohibit the activity if it chose to. So, continuing to allow it then worrying over whether it is suspicious is a self inflicted wound.