There seems to be an issue within MS Graph Explorer such that it does not properly add the permissions eDiscovery.Read.All and eDiscovery.ReadWrite.All when requesting for token. I am able to get this working with the scopes using tokens generated for a APP using MSAL (not using GE).
The token that has these scopes can access the APIs you are trying to use, so I suggest you get a token from AAD using MSAL to use these APIs until the behavior with GE is gone. See screenshot.