I appreciate you sharing details of your concern and the steps you've performed to help isolate the issue, @Jczepiel.

I know a possible reason why you don't have the option to revert to the new invoice experience in QuickBooks Online (QBO). Allow me to provide a more detailed explanation.

It's possible that you unintentionally switched back to the old invoice layout, which would explain why you no longer have the option to return to the new experience.
 

According to the latest update, customers who have switched back from the new invoice experience to the old layout will no longer be able to revert to the new layout. The ability to switch between layouts is still being rolled out, so I recommend waiting for the feature to be updated and automatically added to your QuickBooks account.

For the time being, you might consider exploring this website to acquire details regarding our latest improvements and advancements: New features in QuickBooks.

 If you need further assistance or have additional invoice queries, please let me know by leaving a comment below. I will make sure to respond and provide you with the necessary help.