When mapping is saved, it is stored into a property of the FormContainerBlock. So it should be there when you do restart/rebuild. Do you have custom the editor for that mapping? Or do you return consitent data from your ExternalFieldMappingService?

The following is the class used for the mapping/autofill:

public class CustomerDataAutofill : IExternalSystem, IAutofillProvider
    {
      
        public string Id => "CustomerDataAutofill";

        public IEnumerable<IDatasource> Datasources
        {
            get
            {
                var datasource = new Datasource()
                {
                    Id = "ContextCustomerInfo",
                    Name = "Kontextbaserad kunddata",
                    OwnerSystem = this,
                    Columns = new Dictionary<string, string>
                    {
                        {"FirstName", "Förnamn"},
                        {"LastName", "Efternamn"},
                        {"EMail", "E-post"},
                        {"Phone", "Telefonnummer"},
                        {"Address", "Adress"},
                        {"PostalCode", "Postnummer"},
                        {"City", "Ort"},
                    }
                };

                return new[] { datasource };
            }
        }

        public IEnumerable<string> GetSuggestedValues(IDatasource selectedDatasource, IEnumerable<RemoteFieldInfo> remoteFieldInfos, ElementBlockBase content,
            IFormContainerBlock formContainerBlock, HttpContextBase context)
        {
            if (selectedDatasource == null || remoteFieldInfos == null)
            {
                return Enumerable.Empty<string>();
            }

            if (!this.Datasources.Any(ds => ds.Id == selectedDatasource.Id))  // datasource belong to this system
            {
                return Enumerable.Empty<string>();
            }


            var activeRemoteFieldInfo = remoteFieldInfos.FirstOrDefault(mi => mi.DatasourceId == selectedDatasource.Id);

            if (activeRemoteFieldInfo == null) return Enumerable.Empty<string>();


            var currentUser = ServiceLocator.Current.GetInstance<ICurrentUser>();
            var restService = ServiceLocator.Current.GetInstance<IRestService>();

            switch (activeRemoteFieldInfo.ColumnId)
            {
                case "FirstName":
                    return new List<string> {
                        currentUser.FirstName
                    };

                case "LastName":
                    return new List<string> {
                        currentUser.LastName
                    };
                case "EMail":
                    return new List<string> {
                        currentUser.Email
                    };
                case "Phone":
                    var phonenumber = restService.GetContactById(currentUser.ContactId).Telefonnummer1;
                    if (string.IsNullOrWhiteSpace(phonenumber))
                        phonenumber = restService.GetContactById(currentUser.ContactId).Telefonnummer2;

                    return new List<string>
                    {
                       phonenumber
                    };
                case "Address":
                    return new List<string>
                    {
                        restService.GetContactById(currentUser.ContactId).Folkbokforingsadress.StreetAddress
                    };
                case "PostalCode":
                    return new List<string>
                    {
                        restService.GetContactById(currentUser.ContactId).Folkbokforingsadress.ZipCode
                    };
                case "City":
                    return new List<string>
                    {
                        restService.GetContactById(currentUser.ContactId).Folkbokforingsadress.City
                    };
                default: 
                    return Enumerable.Empty<string>();
            }
        }
    }

I have tried your class with Forms and the mappings are still there when I rebuild/restart the site. Are you running custom code to clear/create new database?

Reason for the failures is unknown. We believe a faulty synchronization between servers could have been a reason but we are not really sure. It is working now anyway. :)