It looks pretty much like there is an 8.0.4 bug in the method GetUsersAdvancedSearch.  I am executing the following where PortalID = 0, whereNames='PreferredLocale' and whereValues='en-US' and getting 0 records in the filteredUsers list:

            List<UserInfo> filteredUsers = (List<UserInfo>) UserController.Instance.GetUsersAdvancedSearch(PortalController.Instance.GetCurrentPortalSettings().PortalId, -1, -1, -1, -1, true, -1, 0, null, false, whereNames, whereValues);

What is most interesting is that if I directly execute the stored procedure associated with this method, I do get the results correctly:

USE [dnndev.me]

GO

DECLARE @return_value int

EXEC @return_value = [dbo].[GetUsersAdvancedSearch]

@PortalID = 0,

@UserId = -1,

@FilterUserId = -1,

@FilterRoleId = -1,

@RelationshipTypeId = -1,

@IsAdmin = true,

@PageSize = -1,

@PageIndex = 0,

@SortBy = NULL,

@SortAscending = false,

@PropertyNames = 'PreferredLocale',

@PropertyValues = 'en-US'

SELECT 'Return Value' = @return_value

GO

I'm not sure how to resolve, but it honestly should not be this hard to run a query of the users.