That's by design. If Spotify client detects there isn't enough screen width, it will not display Friend Activity bar even when it's enabled in settings.
You can also test this on your landscape monitor - try modifying Spotify's width by pulling on either of its sides. At some point when the window is narrower, the Friend Feed disappears, and when you pull the window wider, it reappears.
Hope this helps, have a nice day 🙂
SebastyRock Star 18
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.