A friend of mine just stumbled upon the same issue and I made a quick fix for him that I think might help somebody else:
add_action( 'wp_footer', 'fix_flatsome_checkout_quantity' );
function fix_flatsome_checkout_quantity() {
?>
<script>
jQuery(document).ready(function($){
// fix flatsome checkout quantity
jQuery(document).on('updated_checkout', function() {
jQuery('.quantity').addQty();
})
});
</script>
<?php
}
This works with Flatsome. jQuery('.quantity').addQty();
is what it calls in order to initialize those +/- buttons. You can add it into your theme’s functions.php or wrap it into a small custom plugin.