It turns out the issue was with the roleID's. These were not valid roles in the project, but Autodesk currently provides no clear means of finding the roleID's for a given project and the docs for these requests do not mention how to retrieve the roleID's. Luckily, someone else here recently had the same issue and found out that the roleID retrieval endpoint for BIM 360 still works for ACC (for now at least), so this can be used to retrieve the roleID's to make sure that they are valid in this request. What's clear though is that the Autodesk docs seriously need to be updated and that this endpoint in my original question is missing some proper error handling.