I don't know how to repeat this issue myself. All I would know to do is verify that all assignments appear as they should in the assignments tab of the course, and then insert in the /api/v1/ to the url that appears in your browser address bar and observe the JSON code that gets spit back from the API from within the browser. When I do this, I cannot see any errors as you describe.
Can you repeat this with other courses you might have access to? If so, this seems like an issue that is more easily solved by submitting a case to Instructure support. If your institution is a cloud customer, they should have access to your instance and try to repeat. If you are self-hosted, the developers have made several channels of support communication available on their public GitHub page.