Correct, this is Andre….Anyway, I still cannot replicate what you are experiencing, so I am not sure what is causing the menu to hide. You mentioned that you removed the things you did in the header, but did you put it back to the original state of the theme?
What I’m seeing is that the menu container is collapsing, so something is definitely creating a conflict somewhere. You have a couple choices here to test this out…
1. Install another test site somewhere, install Circumference, create a menu, then test it in mobile
2. Change from the child theme to the parent theme as the active theme and see what happens
Option two should not delete your theme option settings such as colours etc when you go back to the parent then back to the child theme, but WordPress has done some strange things before.
Question…. your menu is a custom menu you made isn’t it?