We discussed this in the slack channel (#api @ https://netapppub.slack.com), but for anyone reading this thread not involved in Slack, here was the answer:
Robert:
Something to check. Were these relationships created through the REST API or via some other method? If it was some other method, do they have a policy-type of async-mirror or vault? If so, they are not supported in the REST API (and won't be returned) until you upgrade to 9.7 where they are supported.
If this applies to you, you can either upgrade your cluster to 9.7 or you can use the CLI passthrough until you get to a supported version. Using the CLI passthrough is documented in the API docs under "Using the private CLI passthrough with the ONTAP REST API".
Kircho:
I have policy-type set to "async-mirror". We just upgraded to 9.6p6.. I will give it a try with the CLI passthrough. Thanks helping out Robert!