idk if this is a glitch or not, but the explore button disappears when you enter a topic or thread.
It's a misfeature. What should happen is that at the top of the screen should be exactly the same navigation bar as on the home page of the site, and below that should be the Discourse navigation bar. There is a Discourse plugin that does the opposite of that, letting you put your own bar underneath the Discourse one, and also removing yours when you move past the beginning of the thread.
So instead of that, we have some extra fields stuck in the Discourse nav bar. But when you scroll down past the title of the thread, Discourse wants to put the title in its nav bar. And that potentially takes up a lot of space, so some of the site navigation is pushed out.