I have a site that I am working on, on the live site this works with Joomla 3/HelixUltimate1.x, the site I am working on is Joomla4/HelixUltimate2.0.11. For some reason I cannot get the Menu item which is defined as a Menu Heading to underline when a sub menu item is selected. My client does not want these top menu items as links so I can't use a system with with a # or link to the first submenu item.

This works when the top menu item is a normal menu item, but not when it's a menu heading. The site is: https://cdpro.mike-underwood.com/ the only Menu Heading (currently) is for CORPORATE EXHIBITION so if you select one of the menu items under that drop down for example https://cdpro.mike-underwood.com/corporate-exhibition/custom what should happen is for CORPORATE EXHIBITION to underline so the user knows they are on a submenu item under that. I have tried on line 115 in the custom css the following and several similar things but I just can't get it to work.
.sp-menu-heading .li.sp-menu-item.sp-has-child.active > a { text-decoration: underline; }

Could please tell me what I am missing. Thank you, Mike