That issue caused when your sticky menu doesn’t get a space while scrolling. You have added this content on about us page:
New and better ways of living to inspire your life. Avant Private Communities brings concepts to life for today’s demanding Senior Living clientele.
Watch for updates on new communities coming soon to a location near you.
which occupied less space and doesn’t provide room for scrolling sticky menu.
If you are not adding more content on about us page then you need to add following code on child theme style.css templates:
Note: This will create extra space if you have less content on page. And please use child theme to add custom css, otherwise your changes will lost on theme update.