eyelashes ghosting with custom mask but not global

[Quarrel Kukulcan]

The left half of this picture is what I get if I create a new Alpha Mask with full system blanks for Head, Eye and Hair.

The right half is what I get if, for the Head, I instead supply a custom alpha mask image with all pixels 0% alpha except the lowest reaches of the neck. Most importantly, the part of the head UV where the eyelashes live (the upper right corner of the head UV) is masked out. I've tried both PNG and TGA export for the mask image, and both white and black RGB values for the transparent pixels (which shouldn't matter for this).

The translucent yellow sphere is attached to the Skull point and is actually about a foot behind my actual head here.

Is there any way to stop the eyelashes from alpha-ghosting with a custom mask?