Hi,
What you are saying is not a bug, Its a VMware functionality. where the files which are not connected to VM are left behind on doing the storage vmotion.
This type of functionally is mostly helpfull when there are multiples -delta files (snapshot files) and are not connected to VM. So users do the storage vmotion and delete the unused files from the old datastore.
Hence according to your query , The nvram files which is left behind is not used by the VM and you can delete the same.
