Honestly, I'm not sure. I have all three map packs so it works fine for me, but I gave up trying to get Horde games on the new maps. All I ever seemed to get were games on the Flashback maps.
If you're looking to either play specific maps or get specific Horde achievements (or both!), then your best bet is probably to start a thread (or find an existing one) to ask for help.
Put the days and times you'd like to play, which maps you want to play on and what you're aiming to get done, and ask for volunteers
Good luck!