Fortigate 100D HA LDAP Users Captive Portal authentication timeout don't work as expected

Fortigate 100D, HA active-passive, Captive Portal form-based authentication, Explicit Proxy. Proxy policy require authentication.

Current setting for timeout is 720 minutes and authentication timeout type is hard. But users requested for authentication several times during the day. Why?

I check configuration using CLI, it's right.