Hello Vinay,
Probably not a bug.
Please take a look at this thread:
Basically, you need Snapdrive to reclaim space from this LUN. DOT does not see the space left by the deleted files as free space because when you delete them via OS (Windows), DOT is not informed that it should free the associated blocks. You tell DOT to free those blocks via the space reclamation feature in Snapdrive.
Hope this helps you.
Regards,
Pedro Rocha.