null conditional operator ?. for struct

Class1.cs

using System;

namespace ClassLibrary1
{
    public class Class1
    {
        private static void Main(string[] args)
        {
            // enabled nullable reference type
            A? a = new A(null);
            a?.Name?.ToString();
            a?.MayBeNull()?.ToString(); // compile pass
            a?.MayNotNull()?.ToString(); // compile pass

            S? s = new S(null);
            s?.DateA?.ToString();
            s?.MayBeNull()?.ToString(); // compile pass.
            s?.MayNotNull()?.ToString(); // compile error. CS0023 Operator '?' cannot be applied to operand of type DateTime
            (s?.MayNotNull())?.ToString(); // compile pass. Execute Time Error
        }
    }

    public class A
    {
        public string? Name { get; set; }

        public A(string? name) => Name = name;

        public string MayNotNull()
        {
            return Name; // warning CS8603 Possible null reference return
        }
        public string? MayBeNull()
        {
            return Name;
        }
    }

    public struct S
    {
        public DateTime? DateA { get; set; }
        public S(DateTime? d)
        {
            DateA = d;
        }

        public DateTime MayNotNull()
        {
            return DateA.Value; // warning CS8629 Nullable value type may be null
        }
        public DateTime? MayBeNull()
        {
            return DateA?.AddHours(9);
        }
    }
}

ClassLibrary1.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <OutputType>Exe</OutputType>
    <LangVersion>8.0</LangVersion>
    <nullable>enable</nullable>
  </PropertyGroup>

</Project>