Paid up letter received but still not cleared?

FEEDBACK ON BA***

O8.04.25 I got another email telling me I still owe. I quickly rep**** on the email asking for a detailed invoice with a final amount.

The person rep**** with an invoice. An hour later I received my "paid up letter" from another person stating that I had paid on 05/12/2024 and that it takes 21 working days to reflect.

After work I went to MTN and they told me I'm still not cleared. That I should contact 135 and follow up.

Like I can't understand, if the account was paid up in December and now I have a letter, why do I still need to wait another 21 working days? After 4 months?