I haven't tried all the armor mod types for the arms, but it does give her bare arms for me. I quite like the look, but I think different arm mods will give different looks, or they should. Perhaps it is a bug, but not one I've seen people mention before.
Seems similar to Iron Bull, where some of the armors just modify the belt buckle, or hip pouch, etc. instead of giving him a full chest piece. Not all armors do it, but some do.
I don't think there's an issue with Iron Bull's armor in the chest area. Some of the armors give him Qunari tattoos instead of metal armor, because of that whole thing where they can use magic tattoos as armor. It's like all that Vitaar tattoos that you can use for his head piece.
I noticed this too and I think it's by design.
The arm mods don't add to the armor value but instead modify stats like dexterity and cunning. I think the logic is that the lack of sleeves improve mobility, thus increasing dexterity.
