When Tasks are deleted from your MSP plan they will not automatically be deleted from PPO upon the next import. The reason for this is that PPO has the ability to only import selected Tasks based on which Tasks are selected on the Import Task Wizard view page:
When you've added a Task to your MSP plan and you selected the Import option, the new tasks will automatically be imported as well.
Re-importing project plans
For more information on the mechanisms involved in re-importing your project plan, please see the following FAQ