Thank you for the prompt reply. I'll join the thread and help clear things up for you, @BrianQW.

The Transaction Report of the revenue account you stated above is a separate report from your custom QoQ P&L report. It means you'll need to press the Save customisation button on the Transaction Report page itself, right after you add the Class column. With this, you can directly open the Transaction Report in question from the Custom reports tab.

You can keep this article for future reference: Common custom reports in QuickBooks Online. It lists the usual custom reports other QuickBooks Online (QBO) users utilise, along with the steps on how to generate them. 

Tap the Reply button if you have other questions about custom reports in QBO. I'll be sure to get back to you. Have a great day!