MSTest tests succeed if run in isolation, fail if run with other tests

I've bumped into this really odd situation where, if I run all my tests together, certain tests will fail (about 7 of them). But if I run just the tests inside the class (they are all part of the same class), then they pass. The test project is a Windows Phone 8.1 MSTest, and I tried running it both with Resharper test runner and MSTest test runner and they both show the same problem. Here's my TestInitialize code:

[TestInitialize]
public void Init()
{
     ResolveDependencies();
     var adsApiService = ServiceLocator
         .Current
         .GetInstance<IApiService<ListAdsReply, PublicAdsEndPoint>>();            
     var navigationService = new NavigationServiceMock();
     var mainPageTrackingService = ServiceLocator
         .Current
         .GetInstance<IMainPageTrackingService>();
     var adInsertionTrackingService = ServiceLocator
         .Current
         .GetInstance<IAdInsertionTrackingService>();
     var connectionService = ServiceLocator
         .Current
         .GetInstance<IConnectionService>();
     _windowsApiService = new WindowsApiServiceMock();
     var contactAboutTrackingService = ServiceLocator
         .Current
         .GetInstance<IContactAboutTrackingService>();
     var filtersTrackingService = ServiceLocator
         .Current
         .GetInstance<IFiltersTrackingService>();
            var filtersService = ServiceLocator
         .Current
         .GetInstance<IFiltersService>();
     var messageHelperMock = new MessageHelperMock();
     _mainPageViewModel = new MainPageViewModel(adsApiService, navigationService, mainPageTrackingService, adInsertionTrackingService, connectionService, _windowsApiService, contactAboutTrackingService,filtersTrackingService, filtersService, messageHelperMock);
}

The ResolveDependencies method doesn't do anything special besides registering dependencies in a Unity container, registering it with ServiceLocator.SetLocatorProvider and doing some Automapper configurations. No async code there.

[TestMethod]
public async Task GivenParameterIsProvidedThenFetchDataShouldReturnValidData()
{
    _mainPageViewModel
        .SearchParams
        .Add(new KeyValuePair<string, string>("lim", "5"));
    var searchParams = _mainPageViewModel.SearchParams;
    await _mainPageViewModel.FetchData(searchParams);

    var list = _mainPageViewModel.AdsList;
    Assert.IsNotNull(list);
}

This is one of the failing tests.

public async Task<ListAdsReplyViewModel> FetchData(List<KeyValuePair<string, string>> parameters)
{
    _cancellationTokenSource = new CancellationTokenSource();

    _cancellationTokenSource
        .CancelAfter(Constants.TimeToCancelAsyncOperations);
    AddSearchKeywordToSearchParams();
    var result = await _listAdsReplyApiService
        .GetWithParametersAsync(_cancellationTokenSource, parameters);
    var vm = new ListAdsReplyViewModel
    {
        Ads = new List<AdInfoViewModel>()
    };
    foreach (var listAd in result.ListAds)
    {
        var listAdDto = Mapper.Map<ListAdDto>(listAd);
        var adInfo = new AdInfoViewModel(_navigationService, _mainPageTrackingService)
        {
            ListAdDto = listAdDto
        };
            vm.Ads.Add(adInfo);
     }
     vm.NextPage = result.NextPage;
     vm.ConfigEtag = result.ConfigEtag;
     vm.Sorting = result.Sorting;
     TotalAds = result.ListAdsCounterMap.AllAds;
     return vm;
}

private void AddSearchKeywordToSearchParams()
{
     if (!string.IsNullOrEmpty(SearchKeyWord))
     {
         var searchKeyword = SearchParams
             .FirstOrDefault(x => x.Key == "q");
         if (!searchKeyword.Equals(null))  
             SearchParams.Remove(searchKeyword);
         SearchParams.Add(new KeyValuePair<string, string>("q", SearchKeyWord));
     }
}

And this are the methods under test. The problem seems to happen when foreach((var listAd in result.ListAds) is called. It's like GetWithParametersAsync(_cancellationTokenSource, parameters); is not being awaited, because I get the following error for the failing tests:

Test method App.Tests.Integration.App.Shared.ViewModels.MainPageViewModelTests.GivenParameterIsProvidedThenFetchDataShouldReturnValidData threw exception: 
AutoMapper.AutoMapperMappingException: 

Mapping types:
ListAd -> ListAdDto
Core.Api.Models.PublicAds.ListAd -> Core.Dto.ListAdDto

Destination path:
ListAdDto

Source value:
Core.Api.Models.PublicAds.ListAd ---> System.NullReferenceException: Object reference not set to an instance of an object.

    at Core.Bootstrap.AutoMapperConfiguration.<>c__DisplayClass0_0.<Configure>b__12(ListAd src, ListAdDto dest)
   at AutoMapper.Internal.MappingExpression`2.<>c__DisplayClass57_0.<AfterMap>b__0(Object src, Object dest)
   at AutoMapper.TypeMap.<get_AfterMap>b__40_0(Object src, Object dest)
   at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.Map(ResolutionContext context, IMappingEngineRunner mapper)
   at AutoMapper.Mappers.TypeMapMapper.Map(ResolutionContext context, IMappingEngineRunner mapper)
   at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context)
 --- End of inner exception stack trace ---
    at App.ViewModels.MainPageViewModel.<FetchData>d__34.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at App.Tests.Integration.App.Shared.ViewModels.MainPageViewModelTests.<GivenParameterIsProvidedThenFetchDataShouldReturnValidData>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

Any thoughts?