When you cast Day’s undoing while Bolas’s flipside 3rd loyalty support is out, it doesn’t prevent the opponent from drawing. It is my understanding that this should not be happening.
Before you say it “works that way” … keep in mind that it does prevent Kumena’s Awakening from allowing the opponent to draw. So yes, bolas’s FS 3rd loyalty does prevent draw effects. Why not Day’s undoing?
//Edited Title -Brigby