Snapshot bug?

I'm doing a lot of housekeeping on my NAS which was rebuilt a week ago after file system corruption.

I've been advised to keep as much free space available as possible and keep snapshots to a minimum.

I have many TBs of data that is completely static things like music and videos that are essentially WORM data that will never change. Applying a snapshot on these shares is somewhat pointless, it may as well be read-only, but the share is also home to some data which changes almost daily.

I made a decision to split the share into two, the static data on one and the changing data on another.

I copied the 826GB of changing data from a snapshot enabled share (Share A), to another share (Share B) on which snapshots were not enabled. My available space went down by the expected 826GB. I then disabled snapshots on Share A and deleted all the existing snapshots.

Having verified the copy to Share B was successful, I then deleted the source data from Share A.

I expected my disk used to go down by 826GB. It didn't. I now have 826GB used by snapshots. But I don't have any snaphots on either the source or the target share. But it appears this deleted data is now stuck in an invisible snapshot.

Is there a process that needs to run to clean this up? How do I get my free disk space back? I'm currently doing the same process with another 730GB of data and my available space is falling further. I expected to have got the 826GB back by now and my NAS is now giving me unnecessary low disk space alerts.

Any advice appreciated.

Thanks,

Ben