After fiddling around a bit, it appears that if there is currently a summon, orb, or whatever the fuck else kind of thing out on the screen that can hit you if you don't attempt a LATE throw break (this is important, it's after the first frame of the throw, ie the most common), the late throw simply fails. You can test this with Wara, Vsion, Nero, etc, set the dummy to late throw break and it just can't break stuff if you're throwing them right as an active hitbox for a summon activates.