Your plan is a state sponsored plan and is exempt from ERISA. ERISA has some onerous terms that require repayment in many circumstances. Because your health plan is a state government sponsored plan, it falls outside of the ERISA requirements for reimbursement. Missouri is an antisubrogation state so state law controls your requirement for repayment; not federal law. Therefore since Missouri does not allow subrogation and because your plan is not ERISA controlled, the plan's claim for reimbursement is not a valid claim.
These determinations are made from an exhaustive review of the applicable laws and plan language. Just because you get a bill from your health plan administrator, that does not mean you owe it.
Sometimes you get to give the plan a "nice try" award instead of money.