After upgrading PPM and switching to the new xml driver, projects now remain locked in PPM when they are saved back from MSP. The project remains locked regardless of how much time elapses between saving the project back to PPM and closing MSP.
This did not occur prior to the upgrade when the Legacy driver was being used.
This will happen if the registry did not get cleared out when uninstalling the legacy driver.
The following registry may contain old values from when the legacy driver was being used:
Create a backup of the registry (Select File -> Export, enter a File Name and Save). Then the values, with the exception of the (Default) value, under the MPP lookup registry key will need to be deleted. They can be deleted by selecting the values in the Name column, right clicking and selecting the Delete option.
Alternatively, the MSP interface can be cleanly uninstalled and reinstalled.