This seems to be a problem, when you include _subscriptions.php out of a function (non-global) context, isn't it?

This would also happen then with other skin stub files, I think.

For me I do a foreach loop to make all variables global (im inside a function), then include the _subscription.php in that function.

If redirect_to does become set as a global couldnt you do;
if( ! isset($redirect_to) ) global $redirect_to;
?
But the way i posted above seems to do the job in a simplier way anyhow....

So yer....

Your fix was the right way to do. I've just committed it and also found the same "problem" in _msgform.php and _profile.php.

Thanks for reporting!