Hi @alexarios,
It appears that the theme you are using does not support the mega menu on the mobile version. Instead, it hides the mega menu and displays the navigation menu from your HubSpot navigation menu settings.
You can double-check your HubSpot navigation menu settings and add the missing menus. Navigate to ‘Hubspot settings > Content > Navigation Menus’, find the main menu that your site uses, and check if there’s a missing item for ‘Events’.
For your information, here’s a documentation from HubSpot on how to handle navigation menus.