Using private static members across partial classes [duplicate]

I have partial classes with static readonly fields, and in one of the partial classes I'm creating a static readonly list of all the static readonly members. For some reason, the list ends up with the correct numbers of entries, but all the entries all null.

public partial class Numbers
{
    public static readonly List<string> All = new List<string>
    {
        One,
        Two
    };
}

public partial class Numbers
{
    public static readonly string One = "One";
}

public partial class Numbers
{
    public static readonly string Two = "Two";
}

var all = Numbers.All;

// all.Count() is 2
// all.First() is null
// all.Last() is null

Can anyone help me understand why this is?

I've noticed that if I change the Numbers.All field to be a non-readonly property, the values within the list are as expected, not null. So I'm guessing this is something to do with how the partial classes are read, or something to do with the readonly usage, but I'm really thrown by this.

Switching to a non-readonly property is obviously easily done, and maybe that's a better approach(?), but I'm really curious why using readonly fields doesnt work.