MemoryMarshal.Cast on a struct with array of known fixed size

using System.Runtime.InteropServices;
using System.Text.Json;

Span<byte> buffer = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20];
Console.WriteLine(buffer.Length == Marshal.SizeOf<TestStruct>());
var tstruct = MemoryMarshal.Cast<byte, TestStruct>(buffer)[0];
Console.WriteLine(JsonSerializer.Serialize(tstruct));
Console.WriteLine(tstruct.b);

[StructLayout(LayoutKind.Explicit, Size = 20)]
public struct TestStruct
{
    [FieldOffset(0)] [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.I4, SizeConst = 4)]
    public int[] arr;

    [FieldOffset(16)] [MarshalAs(UnmanagedType.I4)]
    public int b;
}