This is because you have a CSS Rule (below) setting the Z-index to 90.
Z- Indexes have changed in the last version to accommodate the new fixed on top options for the menus.
Just remove the Two z-index:90; properties from your rule below and it should be back to normal
@media (min-width:768px) {
.menu-primary ul.sub-menu, .menu-primary ul.children{
-moz-border-radius:8px;
-webkit-border-radius:8px;
border-radius:8px;
z-index:90;
padding-top:6px;
padding-bottom:6px;
background-color:#FFF;
}
.menu-primary ul.mega-menu li{
-moz-border-radius:8px;
-webkit-border-radius:8px;border-radius:8px;
z-index:90;
}
}
If this is not a Custom CSS Rule you created, @weaver will have to check if it is a Theme one that was missed in the correction.
Then I can give you a temporary rule to override it until a new version fixes it