https://www.gamespot.com/articles/fortnites-embarrassing-breast-physics-in-season-6-/1100-6462109/

Fortnite Season 6 implements numerous big changes in the battle royale mode of the game, at least one of which is completely unintentional on Epic's part: new breast physics. We've reached out to Epic to clarify whether or not they're being completely patched out or simply subdued, but the company is not able to specify its plans right now.

[Update: It seems as if Epic has already taken action to remove this from the game. In a new tweet, it said, "We have disabled elements of our animation system. This means you may see visual issues with some outfits. Thank you for your patience while we work through these issues!"]