Just some info for anyone arriving here.
Safari thinks this is feature. There is a bug report here (let them know you don’t like this “feature” =]).
When you open the keyboard, the browser’s window is moved up and your content is hidden because the window is out of the screen. Other weird behaviors can happen too, like the one the OP showed.
Check this blog post for more details and more examples of weird behaviors (I copied the image above from it): https://blog.opendigerati.com/the-eccentric-ways-of-ios-safari-with-the-keyboard-b5aa3f34228d