Hello, wjd-ma.
The option to automatically apply credits is turned on, which is why it will always apply the credit to a new invoice. I'm happy to share the steps with you on how to turn this off.
You'll want to turn it off from the Account and settings to prevent it from auto-applying the credits:
- Click the Gear icon, then select Account and settings.
- Go to the Advanced tab.
- Click the Automation section.
- Set the Automatically apply credits option to off.
- Click Save then Done.

If necessary, edit or delete the invoice payment transactions with the credit applied for corrections:
- Go to Sales, then click the Customers tab.
- Open a customer profile.
- Find and open the payment transaction.
- Uncheck the credit applied or click More then choose Delete.
- Click Save and close.

If you need assistance handling customer credits, feel free to open and read this article: Create and apply credit memos or delayed credits in QuickBooks Online.
Need help depositing or recording invoice payments? These articles can guide you through the process:
You can count on me again if you have more questions with QuickBooks Online or your customer transactions. Here to help.