The behavior for backups changed in beta 3. See Backup location for more information on that.
Essentially, backups are either store locally or on S3. In your case it’s S3 and since you disabled the cleanup on S3 it will ignore the maximum_backups setting. Backups aren’t stored on your local disk anymore. The “Backups” tab in the admin section will show you the files stored on S3 and deleting a backup on that tab will remove it from S3.