First, your iPad mini A17 Pro apparently does not support Stage Manager - presumably due to its smaller screen size. The iPad User Guide states the following:
Models that support Stage Manager
- iPad Air (5th generation)
- iPad Air 11-inch (M2)
- iPad Air 13-inch (M2)
- iPad Pro 11-inch (all generations)
- iPad Pro 11-inch (M4)
- iPad Pro 12.9-inch (3rd generation and later)
- iPad Pro 13-inch (M4)
For iPad models that do support Stage Manager, iPadOS multitasking supports side-by-side split-screen in Landscape screen orientation only. You’ll perhaps note that the individual App windows, in side-by-side view, are actually the equivalent condensed “mobile” portrait view that you would see when using an iPhone.
iPadOS multitasking fundamentally does not support an over-under multitasking view in Portrait screen orientation - perhaps as re-scaling an App’s native portrait view to a landscape aspect ratio is not supported by many Apps.
Apple do invite submission of comments and feature requests via its Product Feedback portal:
Feedback - iPad - Apple