Oracle Database Discussions

Hello, I have a issue really can't figure it out. Please help me!

My company has a asp.net application. It sets user session timeout to 15 min. All the login users have 1 default role 'connect' and 1 nodefault role 'new_role' with 'set role' command. The roles are stored in Oracle DB. My issue is:

1. when user login and navigates in system, everything works well. But if user leave system idle for about 8 min, then his connection is gone. (I checked from session view).
2. If I change 'new_role' to default role, this issue will not happen and session will time out in 15 min as we set in application side.

Could anyone give me some clue? Why 'set role' is a matter?

Thanks!