But in the end of the day, it doesn't explain how Nanako was able to see the homunculi of other nor how she lost the ability.
Was it that the pitch black drawing were merely her own interpretation of people ? It could be the case as Nakashi pointed out that she previously were wrong when she thought that the young girl was raped by that old fart, right?
She can’t see homunculus anymore, ‘cause she stopped looking at the truth. Homunculus is the true form of human reflected from the one who looks. She chose to have plastic surgery and make money from her body, which is not true anymore.
In the case that she drawn sth she thought that the girl was deflowered by her grandfather, you need to know that seeing the homunculus is like your 5 senses are more sensitive in a way that can “make you see it” and the unconscious part of you “see” it, however, to understand it, you need to use the conscious part to analyse. Therefore, about the sketch of flower, she misinterpreted about what she “saw”, which means she saw the truth/the homunculus but she didnt understand it.