Riot Games released an update to address the issues of the latest changes in League of Legends. Mid-patch hotfix to 12.13 brought nerfs to Gwen, Master Yi, and Sivir. The reasons behind that decision were explained by lead balance designer Matt Leung-Harrison on Twitter.
In Master Yi's case, we wanted to give him more optimizations to be more performant at highrr levels of play and then move power from his more statchecky elements after seeing how it landed. (though we overshoot).
For Gwen and Sivir, the changelists were hard to predict and we're following up accordingly after their strong launches. W duration impact and E CD early being the main nerfs for Gwen and Sivir list being a lot of directionally good changes with just a lot of moving parts.
The hotfix happened only a day after the Patch 12.13 release. With it, the new champion Nilah came to the game. Corki and Volibear were nerfed, while eleven characters received various buffs. The developers also changed the core gameplay, reducing objective bounties.