In the example at hand, I explicitly unmask gtk+ ~amd64 and emerge reports that pango needs unmasking, but instead of stating the actual reason (being gtk+), it reports some application which uses GTK+, like that
pango r.b. glib-networking r.b. libsoup r.b. midori
(I wouldn't even know midori directly depends on soup, it should rather be webkit?!)
If I deselect, say, midori, it goes on saying that
pango r.b. glib-networking r.b. libsoup r.b. webkit-gtk
(with webkit-gtk being in world). If I go on deselect webkit-gtk, too, it just picks the next best application (cellwriter, in my case) and so on...
I don't have the option to give you the full emerge --info, etc, right now. Perhaps someone else who is also affected can provide them. Mark as needinfo if you like.
Anyway, the bug exists.
*** This bug has been marked as a duplicate of bug 419381 ***