I'm puzzled by this and I can only think the upgrade wasn't complete. All I can think of is to run the system file checker to see if it will restore the registry to Win 8 taskmanager.
Right click the start icon bottom left on the desktop, select command prompt (administrator), and type (or copy and paste) sfc/scannow let it run. If it shows that it has corrected something then I'd run it again. Reboot and check TM again.