EF Core 2.0 When property is calculated based on list of entities it requires two SaveChanges()

Parent has a property LastChildrenName and a list of the childrens.

parent.Children.Add(new Children(){ Name = "FirstChild" };
parent.Children.Add(new Children(){ Name = "SecondChild" };
parent.Children.Add(new Children(){ Name = "ThirdChild" };

DbContext.Set<Parent>.Update(parent);
DbContext.SaveChanges();
DbContext.SaveChanges();

During debugging before first call of SaveChanges() the LastChildrenName has a proper value of ThirdChild however after SaveChanges() only new Children row appears in the db and the LastChildrenName column has value SecondChild.

Calling again SaveChanges() right after, in the next line of code causes that parent's column LastChildrenName is updated with proper value.

But it also causes rising the update trigger twice on the Parent table which I want to avoid.

Am I missing an EF Core setting or a way which can be used on Parent model to update in the db only once? I would like also to avoid changing the get method of LastChildrenName property.

public class Parent
{
    public int Id;
    public ICollection<Children> Childrens {get;set;}

    public string LastChildrenName => this.Childrens.OrderBy(x => x.Id).Select(x => x.Name).LastOrDefault(x => !string.IsNullOrWhiteSpace(x));
}

Children model:

public class Children 
{
    public int Id;
    public string Name;
    public Parent Parent;
}