Just fixed it on that site for you, css added for reference:
#main-nav #mega-menu-wrap-main_navigation #mega-menu-main_navigation > li.mega-menu-item,
#main-nav #mega-menu-wrap-main_navigation #mega-menu-main_navigation > li.mega-menu-item > a.mega-menu-link {
height: auto!important;
line-height: inherit!important;
}
Apologies for the delay in my reply here, we have been working with Max Mega Menu to provide better support for the plugin and it’s advanced features, but unfortunately they provided some incorrect code which has caused a few issues here and there, they have since provided us with a reworked implementation.
– Ed