[AllowNull] attribute ignored by Resharper

Resharper version = 2020.2.1

Consider the following code from a .Net Core 3.1 console application:

using System;
using System.Diagnostics.CodeAnalysis;

#nullable enable

namespace ConsoleApp1
{

    class Program
    {
        static void Main()
        {
            Test test = new Test
            {
                SomeProperty = null // R# warning: Possible null assignment.
            };

            Console.WriteLine(test.SomeProperty);
        }
    }

    public sealed class Test
    {
        [AllowNull]
        public string SomeProperty
        {
            get => _someProperty;

            set
            {
                _someProperty =
                    value != null // R# warning: Expression is always true.
                        ? value
                        : "default";
            }
        }

        string _someProperty = "";
    }
}

The [AllowNull] attribute tells the compiler that although the SomeProperty getter cannot return null, 
the setter can accept null.

The C# compiler is happy with this, and does not issue any warnings or errors.

However, Resharper gives two incorrect warnings, as commented in the code above
("// R# warning: ...").

Is this a problem with Resharper, or have I configured Resharper incorrectly?