When cloning a bug, it asks for the product to put it in, but if choosing the main "Gentoo Linux" product, the cloning doesn't happen, and a new bug (no pre-filling from cloned bug) entry is opened instead. All the other products carry on the cloned_bug_id=xxxxxx GET parameter, but the "Gentoo Linux" link appears to lose it. If adding it back manually, then the functionality works, so it's an issue with the enter_bug.cgi template or something.
Finally got frustrated to the point of looking into it. It turned out our template was passing wrong args to 'Gentoo Linux' product -- probably upstream changed the template params, and we failed to update it. Deploying a fix right now.
Yay, it finally works!