How is it an invoice says it's paid when I save and send it. This only happens with one customer

Hi WD-PropertyMaint, 

Thank you for reaching out and welcome to our community!

One of the possibilities is this client of yours has come credit and the auto-application of credit function has been turned on in your QuickBooks Online. For more information, please visit the link below:

https://quickbooks.intuit.com/learn-support/en-au/customer-refunds-and-credits/enter-and-apply-adjustment-notes-and-delayed-credits-in/00/263023

Thank you and please let us know if you require any additional assistance.