Interesting, I assume it's a problem in both Flarum 1.x and 2.x.
It's what happens when a page uses the main "side navigation" of the forum, but isn't actually listed in the side navigation itself. I've had that in some of my own extensions, and I had to add additional code to make the page a menu item only in that situation so that it can be rendered on mobile.
In the case of the Tags extension, this will affect every secondary tag that isn't one of the top 3 most used, because they aren't listed in the side navigation, and don't have any parent tag that would be listed in the side navigation, like all child primary tags have. So another impacted page is https://discuss.flarum.org/t/reported
That's not specific to Discuss, this will need to be fixed in Flarum itself.
Your screenshot also shows another problem that seems to affect all tag pages: the new discussion button and tag subscription buttons are both set as primary control, and end up on top of each other (the pen and star icons in the top right) 😅
