In Safari iOS15, by default, the address bar is pinned to the bottom of the viewport. As iOS 15 Safari floating address bar explains, you can use padding-bottom: env(safe-area-inset-bottom) to ensure content displays above it.
However, if the keyboard is visible, the env vars don't work, because the floating address bar is shown above the keyboard, and it clears the bottom of the viewport. This can cause some content to be hidden below it.
Here is a simple demo of the issue: https://pmusaraj.github.io/ios15-bottom-bar2.html
On iOS 14 and below or on iOS15 with Safari set to show the address bar at the top, you can see the submit button when the textarea is in focus. On default iOS15, you can't, because the viewport height returned by the device does not include the floating address bar.
Update: WebKit bug report at https://bugs.webkit.org/show_bug.cgi?id=229876
