
They never stack in bank or inventory.
They sometimes stack in decon because glyphs dropped in the wild actually have a motif style. You'll get Breton-style glyphs, Argonian-style glyphs, etc. Of course, the style is entirely irrelevant because they are never displayed and have no effect in-game. But they have that vestigial data associated with them. Glyphs of the same type/level/style will stack in the decon UI. This is why you see partial stacking when deconning.