@Sean Bulger Thanks for posting in our Q&A.
For this issue, I have done a lot of research. I find that the request is:
GET https://graph.microsoft.com/beta/deviceManagement/groupPolicyMigrationReports/{groupPolicyMigrationReportId}/groupPolicySettingMappings/{groupPolicySettingMappingId}
It is suggested to try to add the "groupPolicySettingMappingId" and run the request to check if it is successful.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.