Why does an overridden get-only property stay null when set in base class constructor?

The compiler treats this as below; basically, the code in the constructor writes to the original backing field, in TestBase. It seems that yours is not a supported scenario, but… I do wonder whether the language team have considered this case.

BTW: if you ever want to see what the compiler does with code: sharplab.io

public class TestBase
{
    [CompilerGenerated]
    private readonly string <ReadOnly>k__BackingField; // note: not legal in "real" C#

    public virtual string ReadOnly
    {
        [CompilerGenerated]
        get
        {
            return <ReadOnly>k__BackingField; // the one in TestBase
        }
    }

    public TestBase()
    {
        <ReadOnly>k__BackingField = "from base";
    }
}
internal class Test : TestBase
{
    [CompilerGenerated]
    private readonly string <ReadOnly>k__BackingField;

    public override string ReadOnly
    {
        [CompilerGenerated]
        get
        {
            return <ReadOnly>k__BackingField; // the one in Test
        }
    }
}