When starting a multi-release application with Java 9 WebStart the classes located in META-INF/versions/9 will be ignored when the WebStart application cache is disabled. As soon as the WebStart cache is enabled it works fine.

A simple test can be found below, you can also give it a try by executing http://www.jyloo.com/downloads/public/test/multiReleaseTest.jnlp

Test case:
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package mrtest;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

import mrtest.impl.Implementation;

public class Main extends JFrame{
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run(){
         new Main(); 
      }
    });
  }

  public Main(){
    add(new JLabel(new Implementation().toString()));    
    setTitle("MultiRelease Test");    
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setSize(800, 600);
    setLocationRelativeTo(null);
    setVisible(true);
  }  
}

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//Default implementation class
package mrtest.impl;
public class Implementation{  
  @Override
  public String toString() {
    return "Default Implementation";
  }
}

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//Java 9 implementation class in META-INF/versions/9
package mrtest.impl;
public class Implementation{  
  @Override
  public String toString(){
    return "***** Java 9 implementation *****";
  }
}