Late Throwbreaks

[Irysa]

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.

[Tsukasa Hiiragi★]

There might be more to this.  I've seen a couple instances where an active hitbox has allowed a player to either cancel into block or not finish their combo.  I don't remember the specifics, but I've experienced this odd effect in a Kohaku vs Warachia matchup, involving a plant and Nanaya ghost.  Sorry I can't be more descriptive, it seems to be some preventive feature so you can't infinite effects with extra objects like the aforementioned, or orbs. 

What you described seems to be a piece of a larger puzzle.