OKTA SAML Session Timeout Issue

We are using Angular as the frontend and Spring Boot as the backend with OKTA SAML for authentication. We are facing an issue. If the application is not used for 30 minutes. The application is reauthenticated.

My question is why is it reauthenticated after 30 minutes because the default timeout in OKTA is 2 hours and the NGINX timeout is also set to 2 hours?