It would be a breach of fiduciary duty to loan more money to someone with a history of not repaying his/her obligations. But there are no guarantees that any loan will be repaid pursuant to the terms. Again, why should the Plan enjoy greater enforceability rights than any commercial institution? Recall that a plan COULD allow for payments by check, but is not required to do so. So because payment could have been by check, the answer to your question must be "of course it would not be a breach of fiduciary duty."
What you are referring to is a code for a garnishment, is it not?