Hi there,
the Site imported some Customizer > Additional CSS which included adding those lines. Delete this from the Additional CSS:
/* navigation rules */
.main-navigation .menu > .menu-item > a::after {
content: "";
position: absolute;
right: 50;
left: 50%;
top: 5px; /* controls where the line appears on hover */
-webkit-transform: translateX(-50%);
transform: translateX(-50%);
display: block;
width: 0;
height: 2px; /* controls thickness of the line */
background-color: currentColor;
transition: 0.3s width ease;
}
.main-navigation .menu > .menu-item > a:hover::after,
.main-navigation .menu > .menu-item > a:focus::after,
.main-navigation .menu > .current-menu-item > a::after{
width: 40px; /* controls the width of the line */
color: #ffffff;
}