When you perform quick backup, Veeam Backup & Replication creates a partial incremental restore point. Unlike a regular incremental restore point that contains data for all VMs in a job, a partial incremental restore points contains data only for specific VM(s).
A partial restore point is not regarded a full-fledged restore point in the backup chain. From the retention policy perspective, a partial restore point is grouped with a regular restore point following it. When Veeam Backup & Replication needs to delete a partial restore point by retention, it waits for the next regular restore point to expire, and deletes two restore points at once.