This is an odd one, and we’re not exactly sure why it’s happening with a couple of our apps. However, there’s an easy fix:

1. Go to the map, and click on any of the POIs (two times, until you see the full POI details page).
2. Hit the back button in the top-left corner of screen.
3. Your Filter button should have returned and, from what we’ve seen so far, it’s there for good – you’ll never have to go through this fix again.