[DataContract]
public class MyClass
{
    [DataMember(Name = "Key", Order = 0, EmitDefaultValue = false)]
    private readonly Guid? _key;

    [DataMember(Name = "Body", Order = 1, EmitDefaultValue = false)]
    private readonly Dictionary<string, object> _body = new Dictionary<string, object>();

    public MyClass(Guid? key)
    {
        _key = key;
    }

    public void SetValue(string name, object value)
    {
        _body[name] = value;
    }
}

var obj = new MyClass(Guid.NewGuid());
obj.SetValue("name", "my name");
var opt = MessagePack.MessagePackSerializerOptions.Standard.WithResolver(MessagePack.Resolvers.ContractlessStandardResolverAllowPrivate.Instance);
var bytes = MessagePack.MessagePackSerializer.Serialize(obj, opt);
var json = MessagePack.MessagePackSerializer.ConvertToJson(bytes); // just for check that json has _body' values.

var afObj = MessagePack.MessagePackSerializer.Deserialize(obj.GetType(), bytes, opt); // here afObj._body is null, it's a bug