SSL-VPN Settings - Idle Logout

I have this set for 300 seconds/5 minutes, but it never seems to fire and time me out.  I remain connected - even when I'm away/overnight - and am only disconnected after the authentication timeout expires (which is set for 24 hours.)

I'm assuming that some background traffic from various open applications is making me appear active rather than idle, but it would be great to have this work as expected.

Has anyone else experienced this and have a workaround/fix?