Enable Modern Experience Lists in SharePoint-Hosted Add-In Webs
Add-Ins can have SharePoint components such as Pages, Lists and Libraries provisioned in the App Web, however we have found that Lists and Libraries provisioned using an XML schema do not seem to be able to become Modern. On list view pages it locks as Classic, and there is still an erroneous "Exit Classic Experience" link.
My company has published SharePoint Add-Ins (apps), we use a Picture Library for allowing users to customise images used in our app. The link appears on the All Items page for the Library, even in brand-new instances of the App. We have had AppSource fail our app twice for the existence of this link. Ultimately we resorted to removing all links to the library to keep testers from seeing that page, and simply handing out the link whenever a customer asks for access to customise the images.
I hope this can be resolved eventually. It has been an issue for far, far too long already.