I think the backup logs are causing this. The system user creates a PM for every automatic backup, but it doesn’t invite the admins unless there was an error. I think it’s safe to create the PM only when there actually was an error or if it was a manual backup. I’m going to change that.
https://github.com/discourse/discourse/commit/0bc1fa8aa4ea5393193cc5c44adbf7eee24cd395