Yes, this is currently a known issue. When you uninstall the Altiris Agent from a computer, it doesn't automatically update the database to let it know that it's been uninstalled (by switching IsManaged from 1 to 0). Under your purging maintenance, you have an option to retire or delete computers after 30 days of no communication - machines that have had their agent uninstalled will get caught by this policy.
So, the uninstall process is working, it's just not being properly reflected on the server.