Product: Credit card or prepaid card

My card is with Citibank. I paid my card in full per Citis online banking in XX/XX/2020, but have to come learn this did not actually happen and my card is now reported as 60-days past due. Citibanks system incorrectly represented the balance due and was a UDAAP violation. Details follow. 

On XX/XX/20I logged in to Citibanks online banking system to make a credit card payment because I got an email reminder that my card payment was missed. Upon logging in to make a payment, I elected to pay the balance in full. I selected this system generated option, and it stated my balance of {$460.00} which was the amount paid from my bank onXX/XX/20. As I later learned, the Citibank system did not reflect the actual balance due, it was missing a {$20.00} fee, which has now caused a paid off and assumed closed account to be 60 days past due. 

OnXX/XX/20 I assumed my card was paid in full, as that was the selection I made online, and because the account was previously closed, that the card would be closed. 

In XX/XX/2020 I received a system generated email that my card was past due. This made no sense as I paid the card in full in XXXX. I contacted Citibank on 6/27/20 via phone to find out what was going on. They stated I had a {$36.00} balance, of which was {$16.00} of a recently posted late fee ( XX/XX/20 ) and minimum interest charges. I asked the agent where the difference of {$20.00} was and they could not adequately explain it. They assumed it was a late fee that was still in process and therefore not able to request a refund on it. They also confirmed that I paid my balance in full on XX/XX/20. 

I waited for some time to pass to see what else new would post to my account. OnXX/XX/20 I contacted Citibank to find out an answer to the mysterious {$20.00} difference and found out my account was 60 days past due and in a collections status. Prior to getting on the phone I reviewed my statements and saw that a {$20.00} fee was posted onXX/XX/20. This was apparently the {$20.00}. 

The problem is that on XX/XX/20 when the pay balance in full option was selected from Citibanks online banking system, it did not include this {$20.00} fee and it was unknown to me that this was actually missing from the balance. Only through reviewing electronic statement history did I piece it together. Citis own representatives could not explain it on XX/XX/20. 

I spoke to one agent for about 15 minutes onXX/XX/20to confirm what happened and to request resolving matters, getting any balance paid, and removing and credit history reporting for delinquencies since I was not in error. I was transferred to a second agent after 15 minutes, re-explained the situation. The agent even confirmed and stated I see where your balance was paid in full on XX/XX/20. 

So, I paid my balance of {$460.00} on XX/XX/20, as confirmed by a Citibank representative, only to learn that it was not actually the balance and their system did not accurately reflect the real balance of {$480.00}. Citibanks system posting error/issue has now caused damaged to my credit report. I want Citibank to remove the delinquency notices from their reporting to credit agencies as their system was in error and a UDAAP violation.