Issue
Closing a Project also closes its associated Story records.
Cause
"Set Project Stories to Complete" business rule
Resolution
The "Set Project Stories to Complete" business rule below is responsible for setting any associated Stories to "Complete" state upon closing a Project:
https://<instance-name>.service-now.com/nav_to.do?uri=sys_script.do?sys_id=30bb76e187a2130049e94c2c59cb0ba2
The business rule was introduced as a part of the "Project Management V3" plugin which is included as a part of the "PPM Standard" plugin.