We're testing this new functionality with our app. One issue I've discovered is that because Gmail intentionally doesn't support push via the Mail app, sending codes to a Gmail email means users will likely never see this autofill. It does appear if you enter the Mail app, pull new messages, and then quickly switch back to the code entry in your app and present the keyboard.
I'm basically looking for a behaviour correction here from Apple. Perhaps iOS should intercept notifications for the Gmail app (and other notable apps), or provide a way for devs to publish codes to a system API.
As it stands, a large portion of our customers who use Gmail presumably will not be able to use this autofill feature.