Despite what Velen said when he put M’uru’s heart in the well, I can’t help but wonder if there’s still some void lingering or growing in it, in a way echoing the hallowfall crystal. Regardless of that, I also wonder if it’s just Alleria’s unique circumstances that posed the danger to the well, but perhaps regular void elves aren’t void powerful enough to have the same effect 
Probably not though
Or perhaps what happened with her has maybe awakened some void in the heart.