I've read a BL story in wattpad where the uke also have the same situation. If I remember right the disorder is called congenital insensitivity to pain (CIP). They can't feel, they can't smell, they can't taste. Fun fact they can actually feel pleasure (not sure how true this part is I'm simply basing in the uke here and the one I've read in wattpad). Also here's a sad thing that is said in that story, people in this disorder have small life spans. Rarely are they able to live past 30 yrs old. But I'm not really sure if the uke here really does have CIP, because this disorder is hereditary and is inborn but he said he got it from childhood.
Also I find this quite amusing because here even though the uke can't really smell, he could actually "smell" the ML's scent. Meanwhile in the story I read in wattpad, even though the uke can't taste, he can actually "taste" the food cooked by his lover lol. (≧∀≦)
The doctor in that wattpad story said that he can't actually taste but it is the brain projecting a taste making him think that he is tasting something (sorry don't know how to explain this well)
A message to the the one posting. Stop getting confused sweetie, this manga and its characters can't be judged with logic anymore hahaha. I gave up analyzing things a long time ago and is simply reading now to see how this ends (づ ̄ ³ ̄)づ