The CRAs permit creditors to alternately report either the current status or prior monthly payment history profile status as either the days delinquent (e.g., 30-late, 60-late, etc.) or simply as CO if the debt has also been charged-off.
Updated reporting of the current status as CO is the same as reporting a current delinquency status, with the level of delinquency then being determined by the period since initial date of delinquency.
Thus, either your current status or prior monthly payment history status can properly be reported as CO.
Once you pay a delinquent debt, then the updated reporting will no longer be one of delinquency, such as CO or 150-late. It will report as paid of as settled for less, which are both are a non-delinquent status. However, prior monthly payment history status reporting as CO will remain for any month prior to being paid.
Date of last payment can be disputed if inaccuate, but is totally separate from either the current status or prior payment history profile historical status reporting.
I would not speculate as to why they have indicated that you made a payment that you assert you did not make, but if that is the situation, simply file a dispte and get it corrected.
