Based on the documentation we assume, that for a permission of type Specific people, the permission should be listed with the field invitation populated. But a GET request to /users/{user-id}/drive/items/{item-id}/permissions only lists the owner permission and not the additional permission we added for a specific external user.
We discovered this issue, which states that this doesn't seem to work for OneDrive for Business or SharePoint document libraries, but this answer is already over one year old. Furthermore is it unclear what changes were made to the docs, as the docs don't mention this.
Is this still the case? Will that ever be added? Because this doesn't really encourage the use of the Graph API.