A month too late, hey ho. But wouldn't Tang Yi originally have had the name of his grotty adoptive father, who abused him? Would that not be a good reason for TGD to be happy for TY to take his name even if he knew whose son TY really was? (Plus it was Tang Yi's choice, since ZHY doesn't change her name.) And if he did know, maybe the prospect of explaining the whole parentage saga to TY was just too complicated and fraught with danger to TY's stability & happiness, and to LLZ, and potentially the syndicate. Better a fresh start?

(I have no idea if TGD knew or not, it's an interesting mystery :D)