The menu is decorated with classes indicating the hierarchy. One of these is ancestor page of the current page. The class does not have to be styled, but it often is, to indicate the context of the page’s hierarchy regardless of where it is within the menu hierarchy. And sometimes the class is styled to show the context of the current page within the menu without regard to the page relationship.
Menu styling is one of the trickiest things in a theme. You can read the stylesheet to find the menu styles, to see what the theme is doing. It will help you understand how you might want to change it, using Customizer > Additional CSS, or in your child theme.
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