Instant app won't start directly, shows a screen first

I have successfully published a (very simple) Instant App linked to my personal website.

When I click on my website's link, I was expecting the app to start directly, but instead, I first see a screen with 2 buttons: "Open app" and "Open in browser":

Why?

Other instant apps that I tried do not show this screen and open the app directly (the desired behavior).

We call this screen the "speed bump". It's a security feature. You're right in noticing that it doesn't always trigger. But there's nothing you, as a developer, can do to control whether it shows. The intention is for it to be a black box from your perspective.