When I use the following CSS in a card template, only on AnkiMobile the element is displayed at the bottom of the card content instead of being pinned to the bottom of the screen.
If I open AnkiWeb on iOS, it is pinned to the bottom of the screen as on other platforms. So I don’t think WebKit is the cause.
You may need to give html and/or body a min-height of 100vh. Also, be aware that the webview uses the whole screen, so bottom:0 will cause it to display under the bottom bar.