SharePoint 2019 CSOM not saving FieldUserValue - csom

Question

I'm facing a strange Issue when I'm inserting or updating an item in SharePoint2019 list which contains a user column the save completes with no errors but the user column always have empty value,
below is my code
context.Load(context.Web, web => web.Lists);
await context.ExecuteQueryAsync();
List RiskList = context.Web.Lists.GetByTitle("Risks");
context.Load(RiskList);
context.Load(RiskList, r => r.Fields);
await context.ExecuteQueryAsync();
ListItem listItem = RiskList.GetItemById(projectRisks.Id);
List<ListItemFormUpdateValue> listItemFormUpdateValue = new List<ListItemFormUpdateValue>();
if (projectRisks.AssignedTo.HasValue)
{
listItem["AssignedTo"] = new FieldUserValue() { LookupId = projectRisks.AssignedTo.Value };
}
if (projectRisks.Owner.HasValue)
{
User OwnerUser = context.Web.SiteUsers.GetById(projectRisks.Owner.Value);
context.Load(OwnerUser);
await context.ExecuteQueryAsync();
listItem["Owner"] = new FieldUserValue() { LookupId = OwnerUser.Id };
}
listItemFormUpdateValue.Add(new ListItemFormUpdateValue() { FieldName = "Category", FieldValue = GetSPSelectedChoiceValue(context, RiskList, "Category", projectRisks.CategoryName).Result });
listItemFormUpdateValue.Add(new ListItemFormUpdateValue() { FieldName = "Status", FieldValue = GetSPSelectedChoiceValue(context, RiskList, "Status", projectRisks.StatusName).Result });
listItem["Contingency_x0020_plan"] = projectRisks.ContingencyPlan;
listItem["Cost"] = projectRisks.Cost;
listItem["Cost_x0020_Exposure"] = string.Empty;
listItem["Description"] = projectRisks.Description;
listItem["DueDate"] = projectRisks.DueDate;
listItem["Exposure"] = projectRisks.Exposure;
listItem["Impact"] = projectRisks.Impact;
listItem["Mitigation_x0020_plan"] = projectRisks.MitigationPlan;
listItem["Probability"] = projectRisks.Probability;
listItem["Title"] = projectRisks.Name;
listItem.ValidateUpdateListItem(listItemFormUpdateValue, false, string.Empty);
listItem.Update();
await context.ExecuteQueryAsync();
as you can see in the AssignTo and in Owner Columns no mater how I try to save the users values the list will take the default value which is null.
I have made sure that there is values in the assigned to and Owner properties and I have tried using the ListItemFormUpdateValue but with no luck.
Thanks in advance