ApplicationSettingsBase.Save does not save when a setting is changed indirectly by Martin S. Müller

Consider this code:

using System.Collections.Generic;
using System.Configuration;

class Program
{
static void Main(string[] args)
{
     Settings settings = new Settings();
     settings.S = new List<string>();
     List<string> l = settings.S;
     l.Add("one");
     settings.Save();
     l.Add("two");
     settings.Save();
}

class Settings : ApplicationSettingsBase
{
     [UserScopedSetting]
     public List<string> S { get { return (List<string>) this["S"]; } set { this["S"] = value; } }
}
}

After the first save, the property S is changed without accessing settings["S"]. The change will not be saved to the xml settings file. However, if the change is done e.g. settings["S"].Add("two"), it will get writting the the xml settings file.

I am pretty sure this is by design. However, this behaviour is not obvious and I could not find any documentation about this behaviour.