As you have evidently now discovered, you have encountered the iPadOS Shelf...
The Shelf is an integral element of iPadOS multitasking. Unlike iPadOS14 and earlier versions, multi-tasking (including split-screen, slide-over views and the Shelf) cannot be disabled from settings.
iPadOS permits you to have multiple App windows of the same App - each being self contained. The shelf, when triggered, shows a thumbnail of each Window associated with the App. Each App - such as Mail, Safari, Notes, Calendar, etc. - has its own corresponding shelf.
The Shelf is used to quickly switch between open windows or instances of the App by tapping the corresponding thumbnail. Individual windows can be closed directly from the Shelf; just swipe the unwanted thumbnail off the top of the screen.
This article from Appleinsider has a useful explanation of this feature along with many of the updates to multitasking:
https://appleinsider.com/articles/21/08/12/how-to-master-multitasking-on-ipad-and-ipad-pro-in-ipados-15
The iPad User Guide is also a rich source of frequently overlooked information. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
Apple does invite submission of comments and feature requests via its Product Feed portal. For iPad and iPadOS:
Feedback - iPad - Apple
If you would like to see additional flexibility in being able to suppress unwanted features of the User Interface (UI) in a future version of iPadOS, this would be the most appropriate channel through which to make a request.