Collection attributes are not serialized to xml

Question

I have a custom serializable generic collection, defined (a simplified example) like this:

[Serializable]
public class ArrayOfItems<T> : IEnumerable<T>
{
    [XmlAttribute("Setting")]
    public bool Setting { get; set; } = false;

    [XmlAnyElement]
    public List<T> Items { get; set; } = new List<T>();

    public IEnumerator<T> GetEnumerator()
    {
        return Items.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        IEnumerator iterator = ((IEnumerable<T>)this).GetEnumerator();
        while ( iterator.MoveNext() )
        {
            yield return iterator.Current;
        }
    }

    public void Add( T Object )
    {
        Items.Add(Object );
    }
}

I create an instance and serialize it using xml serializer:

ArrayOfItems<int> ints = new();
ints.Items.Add(0);
ints.Items.Add(1);

using (StreamWriter writer = new 
StreamWriter(@"C:\test.xml", false, System.Text.Encoding.Unicode))
{
    XmlSerializer serializer = new(ints.GetType());
    serializer.Serialize(writer, ints);
    writer.Flush();
    writer.Close();
}

And here is the file I get:

<?xml version="1.0" encoding="utf-16"?>
<ArrayOfInt xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <int>0</int>
    <int>1</int>
</ArrayOfInt>

Does anyone understand why the bool Setting property is not serialized?