Maybe I missed something, but I have been using Photo Mechanic for almost 20 years (bought the first version in 2006), and command W has always closed whatever is open. If it's a preview, it closed that. If there is no preview open, it closed the current open tabbed contact sheet. So if I have opened 20 tabbed contact sheets, and I want to close them I just kept hitting Command W.
Also, this is pretty standard behavior on other apps that have tabs (for example, Firefox or Google Chrome). If you open a bunch of tabs and hit command W, they close, one by one.
So this is new, and like most things that break a workflow that I have gotten used to, it's annoying. Of course, I can close a tabbed contact sheet by learning a different command, but that's not ideal, and not the way that the Mac OS X works in other apps, at least in my experience.
Maybe we are using different terms? When you say that command W closes the tab, I don't see it closing the tabbed contact sheets here. What tab are you saying that it closes?