Fang Chuyi was by no means a fool.
In the battle at Snake Mansion, if it hadn't been for Shen Yue's constant vigilance, he would have almost taken Shen Yue down.
What stopped him from pursuing the truth was his inner conflict and also his sense of loyalty.
For Tian Ni'er, he did not hesitate to take a knife wound to kill Leopard, and for the sake of his brothers, he did not hesitate to "betray" Medusa, all to seek justice for his deceased brothers...
He didn't want to suspect Medusa, and he didn't want to doubt Ye Biluo either.
Every time this thought arose, he swiftly brushed it aside.
Brothers aren't meant to be doubted, that was his life's creed. You could say he was foolish, but if there weren't such fools in this unbearable world, it would only be even more heartbreaking.
But upon following Shen Yue's advice and starting to look into the matter, like Shen Yue, he too became aware, since after all, these were things happening around him.