I've the time to check today and for me it's not a bug.
This plugin inject sessions in the DB (members and guests).
1. Putting 10 as max fake guests in the plugin
2. 10 guests sessions will be added in the DB
3. Then put this limit (max fake guests) to 5
4. Plugin doesn't add other sessions, simply wait for the sessions expiration
and add new sessions only when the guests online go down the setting max fake guests.
The plugin doesn't delete any session then you need to wait natural session expiration.