After the modern view came into the picture for SharePoint sites, users/developers have been toggling between the modern and classic experience based on their needs. As we are transitioning from the classic to the modern experience, Microsoft has provided an option in the interface to change the experience as needed. However, there are certain scenarios where the browser keeps the current experience cached in a cookie and brings back the same experience on refresh. This can be frustrating if you are using modern solutions like customized PowerApps forms and have reverted the site to classic mode.
Clicking on “Return to classic SharePoint” changes the experience to classic. Once you are in Classic mode you should get an option to “exit classic experience” . If you do not find the exit option you can open the developer tools and find a cookie stored for the site named "splnu" and delete it.
The information for the current experience is stored in the splnu cookie and deleting it and refreshing the site will revert the experience back to modern. Happy Sharepointing!!!!!