Steps to reproduce:

1. Open http://<your_jenkins_server_address>/configure page
2. Add new GCE instance and set "Windows?" checkbox
3. Change "Windows Username" from default value "Build" to "tester" value
4. Press "Save" button
5. Goto jenkins server and check value in config.xml:              

   cat $JENKINS_HOME/config.xml | grep "windowsUsername" 
   Output example: <windowsUsername>tester</windowsUsername>

6. Open http://<your_jenkins_server_address>/configure page again

   Expected: "tester" value in "Windows Username"
   Actual: "Build" value in "Windows Username"

    
   Jenkins log shows exception mentioned in JENKINS-39598 but other parameters works as expected. For example, parameter "Windows SSH Private Key Credentials" saved and configure page shows changed value after reopen. So looks like exception isn't related this bug.