You would think that Riot would work on the issues the champions with actual invisibility have, with their abilities not working properly. Instead, the game has somehow managed to give Nunu that option again.
League of Legends Nunu & Willump is a so-called "ganking jungler", meaning that the champion does not power farm as some jungle are wont to do. Instead, he perma ganks lanes, to try and get his teammates ahead in gold and exp.
To do this, Nunu's abilities are geared towards tankiness, crowd control, and movement speed, giving him everything he needs to be one of the best ganking junglers in the game. However, due to some in-game interactions, last season Nunu had the option of going invisible when rolling his giant snowball.
In season 12, this is not the case. What is the case, however, is his snowball going invisible should Nunu release it, or get rooted, and allow it to continue ahead without him, with only the basic outline showing where the snowball is going.
This way the snowball still does its job of stunning enemies, with the addition of being harder to avoid, as, well, you can't see it clearly and assume that it was canceled upon Nunu stopping.
The last time this happened, it wasn't fixed until one of Riot's developers encountered the bug in the game themselves, so we can expect Vandril and others to have their fun before Riot do something about it, especially considering that Riot are currently on a break.