POST /api/v2/conversations/interactions/{id}/wrapup returns 409

Getting a conflict error when trying to set the wrap-up code via API after the call ends. The interaction status is already COMPLETED.

POST /api/v2/conversations/interactions/{id}/wrapup
{
 "code": "100",
 "comment": "Resolved"
}

Status 409. Docs say it’s idempotent so I expected it to just accept it. Is there a specific state window I’m missing here?