What problem does this address?

Currently, when you open a submenu, the downward arrow icon does not change. Ideally, it would flip into an upward arrow when the submenu is open.

What is your proposed solution?

Add a CSS transform to flip the SVG arrow icon. For example, using the following CSS for on-click submenus:

.wp-block-navigation-submenu__toggle[aria-expanded=true] .wp-block-navigation__submenu-icon svg {
    transform: scaleY(-1);
}