From the conversation with Yang Zhen'er and Tang Baona, although they "grew up with Old Xia," they don't seem to have a good understanding of his situation.
Why Old Xia's personality is what it is now, why Xiang Kun perceived such an extremely rational, indifferent sensation when he previously sensed the lyrics written in calligraphy, why he could never perceive Old Xia's dreams - these could all point to the same reason.
Autism? Maybe not necessarily. Yang Zhen'er probably doesn't know much about it.
But it is true that Old Xia was hospitalized due to some issues when he was a child - it might be a key point.
Xiang Kun thought while continuing the conversation at the dinner table. Knowing that Yang Zhen'er didn't have a clear understanding of the origins of Old Xia's personality, he didn't pursue the topic excessively so as not to raise any suspicion.
