Why Graph API not providing AAD users last activity correctly

I'm using Graph api to fetch AAD users list

        internal static async Task<List<Users>> GetUsers()
        {
            List<Users> uList = new List<Users>();
            try
            {
                IConfidentialClientApplication confidentialClientApplication = ConfidentialClientApplicationBuilder
               .Create(AppId)
               .WithTenantId(TenantId)
               .WithClientSecret(ClientSecret)
               .Build();

                ClientCredentialProvider authProvider = new ClientCredentialProvider(confidentialClientApplication);

                GraphServiceClient graphClient = new GraphServiceClient(authProvider);
                graphClient.BaseUrl = Constant.BaseUrl;

                var result = await graphClient.Users
                    .Request()
                    .Select("Id,Mail,signInActivity")
                    .GetAsync();
                while (true)
                {
                    foreach (var item in result.CurrentPage)
                    {
                        var user = new Users
                        {
                            Id = item.Id,
                            EmailId = item.Mail,                            
                            AdditionalData = item.AdditionalData,
                            User = item
                        };
                        var lastActivityDictonery = item.AdditionalData;
                        var arr = ToStringArray(lastActivityDictonery["signInActivity"]);

                        var parsedJson = JObject.Parse(arr[0]);
                        var lastSignInDateTime = Convert.ToString(parsedJson["lastSignInDateTime"]);
                        var lastNonInteractiveSignInDateTime = Convert.ToString(parsedJson["lastNonInteractiveSignInDateTime"]);
                        user.LastLoginDate = lastSignInDateTime;
                        user.lastNonInteractiveLogInDate = lastNonInteractiveSignInDateTime;
                        
                        uList.Add(user);
                    }

                    if (result.NextPageRequest is null)
                        break;

                    result = await result
                      .NextPageRequest
                      .GetAsync()
                      .ConfigureAwait(false);
                }
            }
            catch (Exception ex)
            {
                log.LogError(ex.ToString());
            }

            return _userList;
        }

        static string[] ToStringArray(object arg)
        {
            var collection = arg as System.Collections.IEnumerable;
            if (collection != null)
            {
                return collection
                  .Cast<object>()
                  .Select(x => x.ToString())
                  .ToArray();
            }

            if (arg == null)
            {
                return new string[] { };
            }

            return new string[] { arg.ToString() };
        }

To find the latest sign-in date, I'm utilizing AdditionalData dictionary results that I obtained from a graph API. But data I'm getting not complete. For the same user sometime, I get results and sometime not and sometimes last login date also get change even if its past date.

Example:
 
I call this method and for X user I got 1st Dec 2022 as last signin date but then next day if I do check again then for same X user it shows 2nd Feb 2023 consider both are past date.

I don't have portal access to verify it so I'm bit confused. Am I doing something wrong in this process?

> Note: I'm using Beta graph API URL

Accourding to your description, I'm afraid signInActivity in user property is what you should use to get the last sign in time.

Please note AuditLog.Read.All permission and Azure AD Premium P1/P2 license is necessary for this property.

By the way, I didn't find a document which explain or mention AdditionalData containing user sign in information. So I don't think it can be a good way to get sign in information in this property.

you can get it as json if you do like this.

await graphClient.Users.Request().Select("id,displayName,companyName,mail,mobilePhone,userType,createdDateTime,Guest,signInActivity").GetAsync();

Then you should get it in AdditionalData if you have premission