Using Luatex engine, ZWNJ and ZWJ chars are rendered incorrectly. If the
font has a glyph for it, it does the expected behaviour (force
non-joining or joining respectively) but the glyph get rendered while it
shouldn't. But if the font has no glyphs for it, it has no effect at
all. See the attached example.

-- 
 Khaled Hosny
 Arabic localizer and member of Arabeyes.org team