Class cannot be found by reflection when usings are inside namespace and not outside. Why?

We have some handlers, these handlers are instantiated in IoC by reflection, because they cannot be referenced. This worked just find until I came with StyleCop and moved usings of some handlers inside namespace. Suddenly, our algorithm could not get handler for particular command. I found out that this seemingly insignificant change did the break.

Handler before change:

using System;
using System.Threading;
using System.Threading.Tasks;

using AccessManagement.Common.Configuration;
using AccessManagement.DirectoryServices;
using AccessManagement.Management.AdContact.Commands;
using AccessManagement.Management.AdUser;
using AccessManagement.{NotForYourEyes1};
using AccessManagement.{NotForYourEyes1}.Enums;
using AccessManagement.{NotForYourEyes1}.Extensions;
using AccessManagement.{NotForYourEyes1}.Models;

using Architecture.Core;

using {NotForYourEyes2}.Common;
using {NotForYourEyes2}.Common.Configuration;

namespace AccessManagement.Management.AdContact
{


    [EnableHandler]
    public class MoveAdContactHandler : IHandleRequest<MoveAdContact, AdUserResponse>
    {
        public Task<AdUserResponse> Handle(MoveAdContact request, CancellationToken cancellationToken = new CancellationToken())
        {

Handler after change:

namespace AccessManagement.Management.AdContact
{
    using System;
    using System.Threading;
    using System.Threading.Tasks;

    using AccessManagement.Common.Configuration;
    using AccessManagement.DirectoryServices;
    using AccessManagement.Management.AdContact.Commands;
    using AccessManagement.Management.AdUser;
    using AccessManagement.{NotForYourEyes1};
    using AccessManagement.{NotForYourEyes1}.Enums;
    using AccessManagement.{NotForYourEyes1}.Extensions;
    using AccessManagement.{NotForYourEyes1}.Models;

    using Architecture.Core;

    using {NotForYourEyes2}.Common;
    using {NotForYourEyes2}.Common.Configuration;

    [EnableHandler]
    public class MoveAdContact2Handler : IHandleRequest<MoveAdContact, AdUserResponse>
    {

Algorithm registering handler to IoC container:

    private static void RegisterBusHandlers(IServiceRegistry serviceRegistry, Func<ILifetime> handlersLifetime)
    {
        var handlerProvider = new HandlerRegistrator((i, t) =>
        {
            serviceRegistry.Register(i, t, handlersLifetime());
        });

        var handlerTypes =
        AppDomain.CurrentDomain
                    .GetAssemblies()
                    .SelectMany(a => a.GetTypes())
                    .Where(t => t.GetCustomAttribute<EnableHandlerAttribute>() != null);

        foreach (var handlerType in handlerTypes)
        {
            handlerProvider.RegisterHandler(handlerType);
        }
    }

To summarize -- handler before change is found well, handler after change is not found at all. This is sort of brain twister for me and my team. Can anyone explain this behaviour please?