public override Task<bool> CheckPasswordAsync(BackOfficeIdentityUser user, string password)
    {
        var userService = ApplicationContext.Current.Services.UserService;

        try
        {

            var userStatus = (userService.GetByEmail(user.Email.ToString()) != null) ? true : false;

            var userLogin =  UmbracoContext.Current.Security.PerformLogin(user.Id);

            var user = ApplicationContext.Services.UserService.GetUserById(id);

            return Task.FromResult(userStatus);

        }

        catch (Exception)
        {

            //throw;
        }
        return Task.FromResult(false);

    }

I am having same issue. userStatus shows true since user exists in database however, performlogin does not happen and return user=null when i check umbTicket status.

                  if (UmbracoContext.Security.ValidateBackOfficeCredentials(loginModel.Username, loginModel.Password))
                {

                    var httpCtxWrapper = new System.Web.HttpContextWrapper(System.Web.HttpContext.Current);
                    var umbTicket = httpCtxWrapper.GetUmbracoAuthTicket();

                    var loginUser = userService.GetByUsername(umbTicket.Name); 

                }

any idea. thanks