I think I discovered the problem. I had set the unit to log to disk, and send logs to FortiCloud. Since I did not subscribe to hosted log retention, logs are only kept for 7 days in FortiCloud. When I tried to view logs on the unit itself, for some reason it only showed the last 7 days.
After changing the setting to not send logs to Forticloud, the unit now show log entries going back 1 year. The unit must have kept logs all this time, it just won't show it because of the send to FortiCloud setting.
Thank you both for responding to my post.