Comment by Lily of the Night on Miowlovers

@Miowlovers here's the reality of his last life:
An illegitimate child
No one to train him=seen as talentless
Young age=naivety
Expelled from the family early=lacks knowledge about certain things, including poinsons. 
Since the poison is undeceted, and no one cares to look at his mana pathways before he was underperforming, there was no way to detect it.