Symptoms

Closing a Project also closes its associated Story records.

Release

Kingston

Cause

"Set Project Stories to Complete" business rule

Resolution

The "Set Project Stories to Complete" business rule below is what is responsible for setting its 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 "Project Portfolio Suite W. Financials" plugin.