I agree! Shen Wei is actually a good liar, in the usual run of things - just as he is actually very good with people in general, and has no issues at all with integrating into human society and acting normal.

But as I see it, he doesn't want to lie to Zhao Yunlan, specifically, for several reasons, among them that he wants Zhao Yunlan to *know* him. So when it comes to this one person, he actually IS a very bad liar. He actually wants Zhao Yunlan to know the truth even while he lies. (If only Yunlan would just agree not to talk about it! But then he wouldn't be Yunlan, and in the end Shen Wei doesn't want that, either. <3)