Authorization failed when uploading to Google Group

Question

I am trying to upload a message to a Google Group using this http call:

https://www.googleapis.com/upload/groups/v1/groups/mygroupname@domain.com/archive?ploadType=media&access_token=XYZ...

(Of course substituting the correct email address for the group, and the actual token). This produces the http error 401 (Authorization failed). The target account is Google Workspace. I am using a service account that has the Google Groups API enabled. I have added this scope to the service account and authorized it for my domain:

https://www.googleapis.com/auth/apps.groups.migration

As a test to make sure I have that scope authorized, I take the token that was generated and run it through this API:

https://www.googleapis.com/oauth2/v1/tokeninfo?access_token=XYZ...

The json returned does have the apps.groups.migration scope (along with others for mail, calendar, etc.). I have no trouble accessing those other data types.

Any idea what I could be missing here?