I am under the impression that the automatic space managment for scheduled backups in Windows Server Backup (2012R2) does not delete backup sets created with "backup once".
Is this impression correct?
Yes,it is correct.In general,Scheduled backup and backup once have different destination path. Automatic Disk usage management feature comes into play when Windows Server Backup detects that the backup target does not have enough space to accommodate the backup while backup is in progress. The way Windows Server Backup creates space for new backup is by shrinking the storage space allocated for snapshots (called diff area). As a result, one or more older snapshots (and hence backup versions corresponding to those snapshots) occupying the diff area that got shrunk get deleted. Before shrinking diff area, WSB determines whether shrinking the diff area can free up the requisite space so the backup can happen. If enough free space can get created, WSB goes ahead with the shrinking and continues with the backup. WSB will not shrink the diff area to less than 1/8 of Target volume size as we do not want to lose all past backups just to accommodate this one. This is why sometimes backup fail with target out of disk space in spite of automatic disk usage management feature in Windows Server Backup.