I think I just figured out, in case anyone else has encountered that issue with portraits going dark, or becoming silhouetted. I think what is happening is that the mini portrait is keying off the environmental lighting conditions of wherever you happened to be standing when you levelled up.
So if you are standing in shadow or a dimly lit area, this seems to carry over into the portrait. It happened to me so many times and was becoming very frustrating, so much so that I stopped and took an hour to experiment. By running around and making sure that I was levelling in a fully lit environment I was able to restore my portrait. It's possible that the same thing is happening at camp depending on whether you're trying to respec at night time vs the day, or you know if you level inside a dark dungeon as opposed to outside in the sunshine.
I think this a consequence of trying to have dynamic lighting for the portrait, and might explain some of the odd stuff that happens with the hair too. Not sure if that helps anyone else, but figured it was worth mentioning.
