I assume that you checked the Window Frame option on Node export. So, here are a few steps which will reveal if it's a bug in C2:
Open the package.nw using 7zip and extract the package.json. Open package.json using a text editor and you should see some settings:
If "frame":true is false and you selected Window Frame during the export then it's a problem with C2 (please submit your example as a bug report - I recall someone else reporting this problem perhaps a month ago...). If your exported package.json looks like the above code then I'm afraid that I've just reached the limit of my understanding!