Safari in iOS 26 has a navigation bar that shrinks while scrolling.

This lets the navigation bar interact unreliably with fullscreen modal dialogs and elements that are supposed to be fixed to the bottom of the viewport.

I found an article related to this here: https://stripearmy.medium.com/ios-26-0-be-prepared-for-viewport-changes-in-safari-e867d7eace43

Sticky Button while stationary:

Sticky button while scrolling:

Fullscreen modaldialog stops at transparent navbar:

Navigation bar shrinks while scrolling and you can see the website behind the fullscreen modal: