Are the fields added on the page or on the nav menu item itself ( Via Appearance -> Menus )?
Have you tried get_field( $item->ID )?
Additionally, the walker method should look like this:
function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
Your current method is missing $item entirely. It may be helpful to turn on PHP Debugging.
https://developer.wordpress.org/reference/classes/walker_nav_menu/
