I had the same issue as I wanted to close a trade on NYSE with MOC order on Friday and it has not filled. I checked the audit trail which proofs I entered the order and send the info to IB. Are they not legally responsible to execute such orders? Is there an alternative to IB? Same low commissions which has MOC orders?
It used to be that they took responsibility and compensated either completely or partially. It might depend on how much commissions you generate -- how valuable as a customer you are.