This is not a bug in nUI and is a reasonably well documented problem that has been occurring for many users since the WoW 3.1 patch.

It is most typically attributed to Cartographer's instance maps. It has also been attributed to some other addons especially those that offer maps inside instances (such as Atlas). I should add, too, that the problem occurs even if you are not using nUI if you have one of these mods installed.

My advice is that you enter an instance with a friend and just hang out inside the entrance and just try distabling mods one or two at a time until you can find the one that's causing the problem. I can only tell you that it is not nUI.

I would also suggest that this is not the correct forum for asking for nUI support. nUI has a very active and vibrant support forum at http://www.wowinterface.com/forums/forumdisplay.php?f=86 -- if you have future problems or questions, I strongly recommend you bring them there.