@Aidan McGill Thank you for reaching out to us, similar ask I have answered in the past on this QnA post - https://learn.microsoft.com/en-us/answers/questions/1462426/api-driven-inbound-provisioning-service-not-popula request you to review the same.
If you still have further questions, feel free to post back would be happy to discuss further on the same.
Please remember to "Accept Answer" if answer helped, so that others in the community facing similar issues can easily find the solution.