OAuth2 access tokens generated by GoogleWebAuthorizationBroker.AuthorizeAsync don't work; previous access tokens still do

Question

In the code below, if there is no access token file for [email protected], the GoogleWebAuthorizationBroker.AuthorizeAsync function creates one. That token is NOT detected as stale by "if (credential.Token.IsStale)", but the await client.AuthenticateAsync(oauth2); statement throws an exception that states: "MailKit.Security.AuthenticationException: '535: 5.7.8 Username and Password not accepted. For more information, go to 5.7.8 https://support.google.com/mail/?p=BadCredentials d82-20020a254f55000000b00dbd9eee633dsm2147601ybb.59 - gsmtp'"

Why would the token that is automatically provided result in an authentication failure? I have a previous working token that still works, even after being refreshed. It's just those now generated by GoogleWebAuthorizationBroker.AuthorizeAsync that don't work. I recently added a Google Drive API to the cloud project. Might that be the problem?

  string[] scopes = new[] { "https://mail.google.com/" };
  UserCredential credential = await GoogleWebAuthorizationBroker.AuthorizeAsync(
        GoogleClientSecrets.FromFile(OAuth2Credentials).Secrets,
        scopes,
        "[email protected]",
        CancellationToken.None,
        new FileDataStore(Directory.GetCurrentDirectory(), true));

  using (var client = new SmtpClient())
  {
     client.Connect("smtp.gmail.com", 587, SecureSocketOptions.StartTls);

     if (credential != null)
     {
        if (credential.Token.IsStale)
           await credential.RefreshTokenAsync(CancellationToken.None);
        var oauth2 = new SaslMechanismOAuth2("[email protected]", credential.Token.AccessToken);
        await client.AuthenticateAsync(oauth2);
     }
     await client.SendAsync(mailkitMimeMessage);
     client.Disconnect(true);
  }