MNP CRD is a copy of the NetworkPolicy, but since NetworkPolicy is a core API, it has extra validations in the api-server, like https://github.com/kubernetes/kubernetes/blob/master/pkg/apis/networking/validation/validation.go.
Recent testing of the `endPort` field emphasized some of the validation differences. But this problem is more generic and should probably be fixed for the whole API rather than one field.
This could be fixed in the API by using CEL validations, but that would be a breaking change and would require a new API version.
Another option is to put the validation burden on the implementations, but considering MNP doesn't have a status field, there is no place to report an error (status field could be added though).
    
Somewhat related discussion upstream https://github.com/k8snetworkplumbingwg/multi-networkpolicy/issues/26 

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