Snippet to hide Multi-Carrier Shipping fallback rates if any other carrier rates are being shown. This will work when fallback rate is based on per unit quantity. Multi-Carrier Shipping Plugin for WooCommerce by PluginHive - https://www.pluginhive.com/product/multiple-carrier-shipping-plugin-woocommerce/

functions.php

/**
 * Snippet to hide multicarrier shipping fallback rates if any other multicarrier rates are being shown. This will work when fallback rate is based on per unit quantity.
 * Created at : 03 Aug 2018
 * Updated at : 03 Aug 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/xadapter/ba600aa9fbf72b5409cf64493070c45b
 */
add_filter( 'woocommerce_package_rates', 'ph_hide_mc_fallback_rates', 10, 2 );

if( ! function_exists('ph_hide_mc_fallback_rates') ) {
	function ph_hide_mc_fallback_rates( $shipping_costs, $package ) {

		$fallback_rate 					= 10;	// Provide the fallback rate
		
		$quantity = 0;
		foreach( $package['contents'] as $line_item ) {
			$quantity += $line_item['quantity'];
		}

		$fallback_cost 	= $fallback_rate * $quantity;
		$total_no_of_mc_shipping_rates 	= 0;
		$total_fallback_rates 			= 0;
		foreach( $shipping_costs as $key => $shipping_cost ) {
			$shipping_method_id = $shipping_cost->get_method_id();
			if( $shipping_method_id == 'wf_multi_carrier_shipping') {
				$total_no_of_mc_shipping_rates++;
				if( (float)$fallback_cost ==  (float)$shipping_cost->get_cost() ) {
					$total_fallback_rates++;
					$mc_fallback_keys[] = $key;
				}
			}
		}

		if( $total_fallback_rates != $total_no_of_mc_shipping_rates && ! empty($total_fallback_rates) ) {
			foreach( $mc_fallback_keys as $key ) {
				unset($shipping_costs[$key]);
			}
		}
		return $shipping_costs;
	}
}