public class Foo
{
    // use Typeless(this field only)
    [MessagePackFormatter(typeof(TypelessFormatter))]
    public object Bar;
}

using System;
using System.IO;
using MessagePack;
using MessagePack.Formatters;
using MessagePack.Resolvers;

namespace BugTest
{
    public class Program
    {
        private static MessagePackSerializerOptions serializerOptions = MessagePackSerializerOptions.Standard
                                                                                .WithResolver(CompositeResolver.Create(
                                                                                    new[] { TypelessFormatter.Instance },
                                                                                    new IFormatterResolver[] { StandardResolver.Instance }));
        // alternative options I tried:
        private static MessagePackSerializerOptions serializerOptions1 = TypelessContractlessStandardResolver.Options;

        public static void Main(string[] args)
        {
            double[] a = { 1, 2 };
            int[] b = { 3, 4 };
            object[] test_args = new object[] { a, b };
            var test_source = new SerTest(test_args);

            var bs = ToByteArraySingle(test_source);
            var test_destination = FromByteArraySingle<SerTest>(bs);

            if (test_destination.Args[0] is double[] a2)
            {
                // All good, TypelessFormatter was able to get the inner types right
                Console.Write("All ok");
            }
            else
            {
                throw new InvalidOperationException($"Args[0] is of type {test_destination.Args[0].GetType().Name}");
            }
        }

        public static byte[] ToByteArraySingle<T>(T obj)
        {
            using var ms = new MemoryStream();
            MessagePackSerializer.Serialize(ms, obj, serializerOptions); // Exception is thrown here
            return ms.ToArray();
        }

        public static T FromByteArraySingle<T>(byte[] data)
        {
            return MessagePackSerializer.Deserialize<T>(data, serializerOptions);
        }
    }

    [MessagePackObject]
    public class SerTest
    {
        [Key(0)]
        [MessagePackFormatter(typeof(TypelessFormatter))]
        public object[] Args { get; }

        public SerTest(object[] args)
        {
            Args = args;
        }
    }
}