Here’s a thought, though.
Why are they called “Warbringers” at all? Jaina’s short reads like it tracks exactly with what we are saying about how the story was supposed to be closer to morally grey. Indeed, its contents are seeming to shape up how Jaina might come to be an antagonist to the Horde and has a hand in starting things up - that she’s learned from her father’s (warmongering) legacy and is ready to take it up herself. In fact, I think it supports our claim, Pellex - Warbringers: Jaina is an artifact of the “original” storyline, and Golden says it was far along in development when she joined. Too far along, then, to change.
Now let’s say we’re Blizzard and have made the decision to do this complete storyline change. Now we’re pretty close to a deadline crunch, and we’re changing everything around to support this storyline change. To make all-new Warbringers animations is impossible, so we cut our losses and work as efficiently as possible. If we can only change one of the cinematics, we’ll change Sylvanas’, because she’s going to be the key figure going forward. In addition, Sylvanas’ animated short has a place in the story that neither of the other two do, like it replaced something that was meant to be a different scene or cinematic and ended up serving double duty. Jaina and Azshara’s cinematics were used as “hype” shorts - Sylvanas’ was used in the game.
I’d say that Sylvanas’ Warbringers short was the one that got rewritten and the others were left as-is. I still think my theory adds up.