That's indeed unusual.
Two options that I have in mind are
- rename/delete the VM's .vmsd (Snapshot Descriptor) file, then reload the VM, and try to "Delete All" snapshots again
- consolidate the snapshots manually by cloning the virtual disk. I'll provide details about how to do this if option 1 doesn't work.
In case option 2 comes into play, please attach the VM's configuration (.vmx) file to your reply.
André
PS: Link for how to reload a VM - https://kb.vmware.com/kb/1026043
