If a block device is claiming space on a thin-provisioned LUN you need to use a SCSI reclaim command on the client to tell Quantastor to free the blocks.
Clients systems do not control this themselves automatically (such as VMWare VMFS 6+) and commands to reclaim the space must be run manually.
VMWare Resources are here:
Note: This is strictly a VMWare operation that goes through every block on the LUN to determine if it is used or free and Quantastor has no ability to monitor or control this function.
You can stop the operation if it is causing an impact on the system and start it again later since it will pickup where it left off on the last run.
You can also specify a percentage to scan and do it chunks if needed.
For additional detail you should contact VMWare for any other options with this command.
For Windows, you need 'sdelete':
For Linux, use: 'zerofree' (preferred) or 'secure-delete' (sfill, read man page). Running 'zerofree <filesystem>' does what you need
For Mac, use the built in 'Disk Utility':