Flexbox item not appearing on Safari (iOS 17.1.2)

I have a Website live on http://chatwithsanta.azurewebsites.net/

It uses Flexbox to show chat messages and a chat message input.

Specifically, the Body is a flexbox container with a message screen containing messages, and a message input form at the bottom.

This layout works in Chrome on desktop, even when Chrome is set to simulate an iPhone 12 Pro layout.

But the form does not appear on my own iPhone.

What should I do?