Using [MemberNotNull] in a virtual override that is called in BaseClass constructor

I have the following code Try it here

using System;
using System.Diagnostics.CodeAnalysis;
    
public class Base{
    protected Base(){
        Initialize();
    }
    
    protected virtual void Initialize(){}
}


public class Derived : Base{
    public string _someString;
    
    public Derived() : base(){
    }
    
    [MemberNotNull(nameof(_someString))]
    protected override void Initialize(){
        _someString = "Test";
    }
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
    }
}

I'm still getting the warning

/home/Program.cs(16,9): warning CS8618: Non-nullable field '_someString' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/home/home.csproj]

Now clearly this field will be fully initialized. However the compiler doesn't realise it. Is there a way to make it understand?

The motivation for this is that Derived is a generated partial class I want the extra members to be automatically initialized. If I just call "Initialize()" directly in the Derived constructor, the warnings disappear, but I want to avoid this step.