My guess its due to the following;
A tab container is by default lazy loading. So when opening the popup page and not having viewed the second tab, the content of the second tab isn't loaded and thus won't be rolled-back.
To test my assumption; open popup, open tab 2, switch back to 1 (tab 2 content is loaded) and execute cancel. If I'm correct, then the Activity will be rolled-back.
p.s. A tab in a popup???