Same thing has happened to me at level up with many characters including the protagonist. I have seen it go completely silhouetted and also down to what seems to be half-lit for the portrait. Light enough to make out the major feature but not see the eyes. Both are frustrating as the only way to fix it is to load a prior save.
When creating a new character sometimes the portrait is darker than other times, occasionally the hair will appear more pixelated.
