I am also facing the same issue. I use Allegro 16.0. After a first level investigation, I found that the drill symbols of all the through hole components placed in the bottom side of the PCB(mirrored components) are appearing in the NCdrill_figure subclass. Even if we add it in the film setup for crating an artwork including the "NCdrill_figure" subclass, the symbols will not be present in the final generated artwork. I had also tried to draw some lines in the "NCdrill_figure"subclass to checkwhether it is ignoring the entire subclass whilecreating the artwork. But, surprisingly, the lines I added manually to "NCdrill_figure"subclass appears in the art work.
That is, only the drill symbols present in the "NCdrill_figure" subcalss is not getting 'exported' to the final artwork. I have no clue how to solve this. I think this is a problem with the tool itself. Allegro 15.7 doesn't have this problem because all the drill symbols are coming in the "NCdrill_figure"subclass only.
NB: There is another post about this subject in the forum at