Main Navigation Border Bottom dont work with Flex for Full width

Hi, before flexbox i had a css for a border for my main navigation and is like this

.main-nav,
#mobile-header {
	border-bottom: 1px solid #e0e0e0;
	-webkit-box-shadow: 0 2px 2px -2px rgba(0, 0, 0, 0.2);
	        box-shadow: 0 2px 2px -2px rgba(0, 0, 0, 0.2);
}

I have set up navigation to full width but the border is not full width, this is happening only with the flexbox option.