I can duplicate this at will... If I have C3D maximized, then after startup I resize the drawing window, the Navcube will flip around and show that I am looking at the Bottom instead of Top. Changing back to Top will change the UCS to an Unnamed UCS.
If I shrink the C3D window, exit C3D, start C3D with a startup drawing (STARTUP=0), the NAVCUBE initially displays correctly, but as the windows resize themselves I can watch the NAVCUBE flip around....see this video
It does this with my DWT and with the OOTB DWT's. With, or without, my add-ins loading. Anyone else seeing this?