So when I heard about the pokérus smiley icon, I looked at my previously pokérused Pokémon. None of them had it. I assumed I was just not looking in the right place (their description on the top screen in the pc), and left it alone.
However, I've been traded four Pokémon with a smiley. Confused, I tried spreading pokérus on my non-smiley Pokémon to see if I̶ ̶w̶a̶s̶ ̶h̶a̶l̶l̶u̶c̶i̶n̶a̶t̶i̶n̶g̶ ̶t̶h̶e̶ ̶p̶o̶k̶é̶r̶u̶s̶ I could spread it again, and it didn't work. They also appear to gain 2x EVs, but I don't know for sure. Out of 5-8 Pokémon that I've had with pokerus, only a few of them got the icon. Some of them were traded over, and others were Pokémon I spread to. None of the ones I've spread to by myself or that came with pokerus had it, only the ones that were already cured when they came to my game.
So, what are some known/confirmed reasons why the smiley icon will not appear? Is there some chance that it will not appear that I don't know of, or is there something wrong?
Edit: Throwing in this. Mew, Empoleon and Diancie had pokérus. Wailord has pokérus.