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Updating fails when using an identity column that is not a key

See original GitHub issue

I define an entity like this:

public class Entity
{
    public Guid ID { get; set; }
    public int ClusteredId { get; set; }
    public string SomeText { get; set; }
}

and a context like this:

public class SomeContext : DbContext
{
    public DbSet<Entity> Entities { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Entity>(b => {
            b.HasKey(x => x.ID).ForSqlServerIsClustered(false);

            // With the following two lines the update FAILS
            b.Property(x => x.ClusteredId).UseSqlServerIdentityColumn();
            b.HasIndex(x => x.ClusteredId).IsUnique().ForSqlServerIsClustered();

            //// With the following two lines the update works
            //b.Property(x => x.ClusteredId).UseSqlServerIdentityColumn().ValueGeneratedOnAddOrUpdate();
            //b.HasIndex(x => x.ClusteredId).IsUnique().ForSqlServerIsClustered();

            //// With the following two lines the update works
            //b.Property(x => x.ClusteredId).UseSqlServerIdentityColumn();
            //b.HasAlternateKey(x => x.ClusteredId).ForSqlServerIsClustered();
        });
    }
}

If i use this context in the following way, it will fail on the second SaveChanges(), because EF will generate SQL to update the ClusteredId column.

Entity entity = new Entity { SomeText = "Some Text" };
context.Entities.Add(entity);
context.SaveChanges();
context.Entities.Update(entity);
context.SaveChanges();

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Entities] SET [ClusteredId] = @p0, [SomeText] = @p1
WHERE [ID] = @p2;
SELECT @@ROWCOUNT;
',N'@p2 uniqueidentifier,@p0 int,@p1 nvarchar(4000)',@p2='...',@p0=1,@p1=N'Some Text'

If, when defining the model, I either specify ValueGeneratedOnAddOrUpdate() for the ClusteredId property, or use an alternate key instead of a unique index on the same property, then the code will work, but in my oppinion either one of those two changes should not be neccessary.