Ah, yes - the default behaviour of the sidebar menu is to hide when you click something in it!
This is controlled by the Javascript in the theme. I believe you’ve switched to the up-to-date Material Design, so if you look at your standard-page.html
you should see something like:
$('.left-nav').off('click').on('click', 'a, button', function() {
if ($('.nav-shield').is(":visible")) {
$('.nav-shield').trigger('click');
}
});
You can delete that whole block to prevent the auto-hiding behaviour. (Or, you can use roles to select which Links and Buttons do and don’t trigger an auto-hide of the mobile sidebar.)