Thank you for posting here in the Community. I can help you clear this setting in QuickBooks Online.
It usually happens when your customer has unapplied credits. If the Automatically apply credits option is enabled, open invoices get paid automatically.
I suggest switching it off to avoid invoices from coming up as paid. Here's how:
Click the Gear icon in the upper-right corner.
Select Account and settings below Your Company.
Go to Advanced.
Scroll down to Automation and turn off Automatically apply credits.
Hit Save.
If you want to view the credits by Customer, there's a report available. I recommend using the Transaction List by Customer report.
Please follow these steps:
Go to Reports.
Choose the All Reports tab.
Go to Review Sales.
Select Transaction List by Customer.
Click Customize.
Press the arrow next to Filter.
Choose the Transaction Type drop-down arrow, then choose Credit Memo.