There was a known issue a little over a year ago where invalid entries would be added to the statistics_media table, causing the database to balloon in size. It was related to the use of the guest account for playback. However, that issue was resolved as of version 1.22.1.4228. You could check too see if yours is accumulating a bunch of records with a null account_id.
Also, I’ve never heard of Plex Media Server itself restoring a backup database on its own. Are you using a VPS or service which might be doing that? Or maybe it’s just a new PMS feature if which I was unaware.
