Apologies for the delay! Our forum email notifications stopped working, so I missed your post....
I've never seen your issue before....it could be because 500000000 is such a large number, maybe there is some kind of overflow in the group comparison logic.
(Note - in nworks 5.7, I have changed the Capacity property for vDisk to be in GB, so it is much easier to handle!)
I'll see if I can repro your problem in our lab here.
Meantime, are there any other criteria you can use to create a group of these VMs with large disks? (in the worst case, I guess you could create a static group....)