First... issues with the XP bar and the pet bar would both have nothing to do with nUI6 as they are both still managed by nUI5.
That said, are you switching in and out of windowed mode by any chance? It sounds like you may be describing the problem with the graphics rendering engine in WoW. The XP bar should only go away if you are level 80 (soon to be level 85) and otherwise there's no explanation for it being hidden other than the graphics bug described here:
http://www.wowinterface.com/forums/s...ad.php?t=25715