This is by design and documented:
[quote:7bapmfwb]Note: backups are only made when pressing the 'Save' button. Simply opening a project that somebody has sent you will not make a backup for it unless you modify it and click 'Save'.
In other words, autobackup means "upon save, make backup if time since last backup is longer than the interval given". Autosave is meant to be the mechanism to trigger that on a regular basis automatically without having to press save, so closing as won't fix.