Some versions of ESXi 6 do not treat VMDKs created by Workstation correctly.
To be precise this is about the format
That format when it is accepted by ESXi comes with a text descriptorfile plus one large file typically named name-s001.vmdk
Warning: this format will prevent automatic backuptools from doing their job and it also prevents you from merging snapshots back into the basedisk.
The behaviour is inconsistent !
ESXi should either (preferably) deny using this format in the first place – which is the intented behaviour – or it should be able to handle it correctly : including the function of merging snapshots.
Most probable reason why you may have this vmdk-type at all on ESXi is manual imported VMs from Workstation or Fusion when the admin is not aware of the inconsistent treatment of this vmdk-type.
According to the VMware documentation this should not happen !