Have you downloaded any reskin mods or something that effects NW directly? Same goes for Warband... if you've done that, it will effect certain resource and texture files and you can end up with botched graphics. For example, a common one you see in NW reskins is downloading a new unit, and because that unit has an altered file, it changes the look or removes parts from other units. That can happen with objects as well. Looks like it happened with your NW... Only thing I can think of (as a non-dev), is to verify your game cache, uninstall, reinstall, and test it again. If the same problem persists, then you'll need to ask one of the devs or a modder like Willhelm, BetaKnight, Rigadoon, etc. They'll know what's up for sure.
