This appears to be a limitation of ZAPI. Looking at the docs for the "destination-cluster" and "source-cluster" properties returned by the API they have this description:
Specifies the destination cluster name for the SnapMirror relationship. The parameters for the name of the destination Vserver, and the name of the destination volume must also be specified if using this parameter. This parameter is available only on:
Data ONTAP 8.1 operating in Cluster-Mode.
Data ONTAP 8.2 or later operating in Cluster-Mode if the relationship control plane is 'v1'.
Why it has this behavior I have no idea...it does seem counterintuitive. Note that the second bullet says "8.2+ if the control plane is v1", my understanding is that this refers to the source being an 8.1 system (or earlier). So, if both source and destination are 8.2+, then the v2 control is used and the fields are not populated.
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