To be able to restore from a forward incremental backup, you need to have a full backup file and a chain of subsequent incremental backup files on disk. If you delete a full backup file, the whole chain of incremental backup files will become useless. In a similar manner, if you delete any incremental backup file before the point to which you want to roll back, you won’t be able to restore VM data (since later incremental backup files depend on earlier incremental backup files).
For this reason, if you select forward incremental backup method, in some days there will be more restore points on disk than specified by retention policy settings. Veeam Backup & Replication will remove the full backup chain only after the last incremental backup file in the chain becomes outdated.
For example, the retention policy is set to 3 restore points. A full backup file is created on Sunday, incremental backup files are created Monday through Saturday, and a synthetic full backup is scheduled on Thursday. Although the retention policy is already breached on Wednesday, the full backup is not deleted. Without the full backup, backup chain would be useless, leaving you without any restore point at all. Veeam Backup & Replication will wait for the next full backup file and 2 incremental backup files to be created, and only then will delete the whole previous chain, which will happen on Saturday.