This is an issue with Dicier Dark versions. Basically, Dicier does the X_ON_DY glyphs by overlapping two glyphs ("X_ON_" and "DY"). That works fine for the fonts where the dice shapes are outlines, but in the Dark fonts the dice shapes are solid/filled, so the font just layers a black number on a black shape.
If you're dead-set on using those exact codes then there's no easy fix (you could probably wrangle something with CSS given your situation), but if you just want the icons then use the polyhedral dice font features specified in the user guide.