Hi there ZenKing,
Could you perhaps try this to doubly confirm? From the ESXi shell run the following:
esxcli storage core device list
Check each of the devices and if any have the Thin Provisioning Status set to yes - then you have a thin provisioned LUN and would explain the UNMAP command working. If this shows as unknown then it is a thick provisioned LUN.
Did that help or has it added to the confusion