Traffic log show different user for same ip via FSSO

We have set up agent-based FSSO for logging the user AD logon and the traffic in the FortiGate. The logon user can be shown in the FortiGate traffic log. However, we found that when the 1st user log out the device and 2nd user log on the same device. There are two users (1st and 2nd) shown on the traffic log for the same IP at the same time. Even the former user has been logged out for over 15 mins. It seems there are some sessions still caching the former user logon. Is there any suggestion on it?

Thanks,