After a discussion with Julien Cristau (Debian maintainer), Markus Kuhn (xorg maintainer) and Constantine Stathopoulos (designer of the glyph), I realize that "incorrect" was wrong: the form is used occasionally in modern Greek print and often in handwriting. However I still think the more common print form should be used. I have reported this as xorg bug 25810: https://bugs.freedesktop.org/show_bug.cgi?id=25810