Do all of the LEGO Minifigures Series 14 Female Ghosts have a smaller, more square-ish neck part?
I asked with my BIONICLE design friend, who asked his manager, who asked a guy on production, who... honestly, I don't know how many people were in the stream, but the answer came back as one word: Yes. That's all I know.
It's not a very detailed answer, I'll admit, but it is a true/false question.
I just checked a random handful of my collectible minifigs. Most of them have no black mark at all. Some of them, such as the Siren you posted, have a small black mark. Other recently produced minifigs do or don't have the mark: my Dr. Who figs have the mark, the Ghostbusters Firehall figs don't. Only the collectible minifigs seem to make use of the smaller mark, as far as I can tell. The standard figures have a larger mark.