I'm assuming you opened a support ticket because Known Issue: DT471022 was approved the other day.
Looking at step 3.3, that's an objectively wrong step. I'm not sure that it's leading to your issue, but the REST API uses the object name for sending & receiving child objects in the object structure, not the relationship name. By replacing the object name with the relationship name I could see it impacting the upload. I would try to revert that change and see if that resolves your issue.