It's to stop a preview from showing up in the recents screen. Try loading a page in incognito mode and then pressing the overview button. Notice that you just see a grey square instead of the page you loaded.
They should be but on android, it's bot handled by the same setting.
So as an app developer I can either set the `FLAG_SECURE` to hide both or I need to create own workarounds like navigating to the root view element and setting it to invisible whenever the app is paused just to hide it from the recent apps.
