EntityFramework Core 2, update date in every update

I have a scenario where my template has the fields CreatedOn and UpdatedOn and following what I found in the EF Core documentation I implemented my class as follows

builder.Entity<Registro>(b =>
        {
            b.Property<DateTime>("CreatedOn")
                .IsRequired()
                .ValueGeneratedOnAdd()
                .HasDefaultValueSql("GETUTCDATE()");

            b.Property<DateTime>("UpdatedOn")
                .IsRequired()
                .ValueGeneratedOnAddOrUpdate()
                .HasDefaultValueSql("GETUTCDATE()"); ;
        });

public class Registro
{
    public Guid Id { get; set; }
    public string Nome { get; set; }
}

In insert both fields are filled with the correct value, however in the update does not happen as expected that would update the field with a new date. It also follows how I am doing the entity update

public void Put(Guid id, [FromBody]string value)
    {
        var r = context.Registros.SingleOrDefault(x => x.Id == id);

        r.Nome = value;

        context.SaveChanges();
    }