While defending my title of champion in the arena I have seen that a Stone Giant Lord uses the same sprite as a regular Stone Giant (I uploaded a screenshot).
Looking in the sprites folder (that in the linux version is ~/.steam/steam/steamapps/common/ADOM/gfx/adom/NPC) I have seen that both the sprite 36.png and 251.png represent a regular stone giant (and the same happens for the female versions). I think that this could be the source of the bug. This was in the linux version of the R67 beta on steam.