On a maven2/3 project:

• Post-build actions
  • Post build task (checked)
    • Add a Task
      • Save
        > RESULT = Post build task saved and works as expected

Then on the same job, go to configure:

• Post-build actions
  • Post build task (already checked)
    • 'Add another task'
      • Save
        > RESULT = This second Post build task is not saved. This task is lost when you go back to the job configuration