Can you please confirm that after reboot there is a change in path cost of port cost. As you have manually configured the port cost is should not change for that switchport but yes path cost may change after reboot as switch may found some other best path to reach the root bridge. All depends on topology and your network design.
Just want to confirm when the switch is rebooted does the value you have configured in the command changes in your config or you are checking this with a "sh spantree" command?
