I can share some information about the customer credits in QuickBooks Online.
May I know how did you create the customer credit? Did you create a credit memo or a delayed credit? These transactions has different effects on your reports and customer balances.
When you create a Credit Memo, it decreases your customer’s existing balance and increases your income account in the P&L report. While Delayed Credit will only affect your client's balance once it is applied to an invoice.