I’m assuming you’re following this documentation on how to obtain access tokens generated as part of an authentication flow that involved an external identity provider, in this case, Facebook.
If that’s the case, have in mind that the key part is that the Management API access token you use to call /api/v2/user/{user-id} needs to have the read:user_idp_tokens scope. More specifically, if the access token only has the read:users scope the request will still succeed, but you just won’t have access to certain properties related to the issued token, being the access_token property one of the ones that would not be returned.
The recommendation would be for you to ensure that you’re calling the API with a suitable access token to retrieve the information in question; if you confirm you have the right scopes but still experience an issue then you should update your question with additional information on how to reproduce the issue.