I entered the dungeon on my draenei shaman and had my helmet hidden, as soon as you enter you get the buff “human illusion” which instantly put my helmet back on display. No matter what I did, unless removing the helmet itself, I could not hide it. I tried this even after turning off all my addons and the bug still remained the same. As soon as I leave through the instance portal, the helmet goes back into hidden. Not sure what to do about this one. I was merely curious about my human appearance :<
I would suspect this is because that dungeon was introduced before the ability to ‘hide helm’ was a thing. Consequently it’s just ignored when your appearance is changed to be human.
It’s certainly a bug, but I would suspect quite low on the list of ‘things to do’ 