It's likely that's because it currently uses the built-in Android WebView (which may store its own pseudo-history). That is a problem, but I believe their plans to replace WebView with Gecko will likely solve this problem. https://github.com/mozilla-mobile/focus-android/issues/13
- Erase your history.
- Go to HN, click any link you haven't clicked before.
- Wait for it to load.
- Erase your history. Make sure you see the notification "Your browsing history has been erased".
- Go to HN again, and see the link you've just clicked still highlighted as 'visited'.