I honestly cannot understand it at all. I feel like the whole world is a fucking joke at this point with zero sense. You have them act modest by covering their cleavage with their hands when bowing, put blankets over their legs, but while performing it's allowed to show it all? What is the difference, if they sit down during a show/ interview they are also doing their job just like performing. Matter of fact when they sit down you would see less than when twirling around so their skirts and whatnot goes up.
I think in Nayeon's case she took the picture but didn't think that much cleavage would have been shown so she quickly used an emoji to cover up, I think in the last pic she also cut the pic off right where it would show some cleavage. Most idols also take several pics in the moment, then later upload them on their sns. If it would be bad they would either cut the part they don't like or use stickers to cover parts or people up they don't want to show in the pics. Apparently in JYPe they would also check before members can post pics, but I doubt this applies with Twice as they are 30 or nearing their 30s, plus veterans.