That feature is called "reachability" and is designed to let the user access controls at the top of the screen with their thumb. It's a nice idea but triggers unintentionally too often IMO.
Thank you! I couldn't remember the name of it (and didn't feel like digging through menus to find it), so I haven't been able to disable until just now!
https://support.apple.com/guide/iphone/use-reachability-iph1...