I did nothing special, not changed anything in the Unraid fundamental...
I remember that it was a Plugin that did that (I think proftpd, but not sure.) I reboot the Unraid when I upgrade version, so I did reboot it a lot since all beta version. This problem is present since beta2 or beta3. It never emptied itself.
If you say it reset itself at powerdown, there is something that fill it, but when I boot the server, it's already at 100%...