Thanks Jason Spence's recommendation. I use okta.signOut() after oktaBaseUrl/logout, works fine for me.

signout(): void {
  const oktaBaseUrl = `${environment.okta.issuer}/v1`;
  const oktaTokenStorage: any = JSON.parse(localStorage.getItem('okta-token-storage'));
  const oktaIdToken = oktaTokenStorage?.idToken;

  window.location.href = `${oktaBaseUrl}/logout?id_token_hint=${oktaIdToken.idToken}&post_logout_redirect_uri=${environment.okta.logoutUri}`;
  
  this.okta.signOut();
}