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I withdraw the question – I needed to include the rest of the code on that page, i.e.,
/* Apply Hover Styling to active Flyout Links and ancestors */
#{$wrap} #{$menu} li.mega-menu-flyout ul.mega-sub-menu li.mega-menu-item.mega-current-menu-item > a.mega-menu-link,
#{$wrap} #{$menu} li.mega-menu-flyout ul.mega-sub-menu li.mega-menu-item.mega-current-menu-ancestor > a.mega-menu-link,
#{$wrap} #{$menu} li.mega-menu-flyout ul.mega-sub-menu li.mega-menu-item.mega-current-page-ancestor > a.mega-menu-link {
@include background($flyout_background_hover_from, $flyout_background_hover_to);
font-weight: $flyout_link_weight_hover;
text-decoration: $flyout_link_text_decoration_hover;
color: $flyout_link_color_hover;
}