Hello,
I think the footer nav is only available on iPhone. On iPad, the footer nav is on the top. But on Android device I think what you see is maybe the Discourse Tab Bar for Mobile. This component hides the footer nav on iPhone. Or as Robert mentioned the Layouts Plugin has also own footer navigation bar.
The Sidemount forum seems to using my theme component: Floating Navigation Controls and Desktop Navigation Bar for Mobile where I hid the footer nav on iPhone. So that is the expected behavior on that case. 