Google will now have AMP trump App URLs in the Google mobile search results.
That means if a website has both AMP URLs enabled and also has a mobile app with Firebase App Indexing formerly Google App Indexing , Google will show the user, even if they downloaded your app, the AMP URL over the deep link to the page within your app.
Google quietly posted this in their data anomalies page, where they wrote:
Search results on mobile devices have begun to direct users to the equivalent AMP page, in preference to the equivalent app or web page.
This might result in a decrease of traffic to your app pages when an equivalent AMP page exists.
Google s John Mueller confirmed this for me on Twitter saying, Yes, at the moment if an AMP exists for a given URL then we will send the user to that AMP not the deep link.