Checkout Block: Prevent changes in the selected shipping method when new rates are added or removed

src/Shipping/ShippingController.php

@@ -77,11 +77,36 @@ function() {
 		add_filter( 'wc_shipping_enabled', array( $this, 'force_shipping_enabled' ), 100, 1 );
 		add_filter( 'woocommerce_order_shipping_to_display', array( $this, 'show_local_pickup_details' ), 10, 2 );
 
+		add_filter( 'woocommerce_shipping_chosen_method', array( $this, 'prevent_shipping_method_selection_changes' ), 20, 3 );
+
 		// This is required to short circuit `show_shipping` from class-wc-cart.php - without it, that function
 		// returns based on the option's value in the DB and we can't override it any other way.
 		add_filter( 'option_woocommerce_shipping_cost_requires_address', array( $this, 'override_cost_requires_address_option' ) );
 	}
 
+	/**
+	 * Prevent changes in the selected shipping method when new rates are added or removed.
+	 *
+	 * If the chosen method exists within package rates, it is returned to maintain the selection.
+	 * Otherwise, the default rate is returned.
+	 *
+	 * @param string $default        Default shipping method.
+	 * @param array  $package_rates  Associative array of available package rates.
+	 * @param string $chosen_method  Previously chosen shipping method.
+	 *
+	 * @return string                Chosen shipping method or default.
+	 */
+	public function prevent_shipping_method_selection_changes( $default, $package_rates, $chosen_method ) {
+
+		// If the chosen method exists in the package rates, return it.
+		if ( $chosen_method && isset( $package_rates[ $chosen_method ] ) ) {
+			return $chosen_method;
+		}
+
+		// Otherwise, return the default method.
+		return $default;
+	}
+
 	/**
 	 * Overrides the option to force shipping calculations NOT to wait until an address is entered, but only if the
 	 * Checkout page contains the Checkout Block.