My take on it. It happens and those that are very bad at it are probably unaware. Address people, on the spot, when they do it. Their technique can be improved to lessen the impact on the riders behind them, if they are willing or encouraged to listen. You can’t just say it happens and it’s a part of cycling so live with it. In the extreme cases it’s very dangerous. I’ve sat behind plenty of riders that don’t noticeably do it while transitioning. They pick the correct gear selection before the climb and keep pedalling as they transition out of the saddle. People may slow gradually but that’s different almost stopping and throwing the bike backwards in order to get into a more comfortable position for climbing. In a large group ride think of the riders behind you and keep them in mind with whatever you do on the bike.