It's also worth noting that...


...the lightning disaster that hit Sphene's shard cannot have caused it to be rejoined, as she mentions that the dome was constructed after that point.

That's not to say that it couldn't have been rejoined at a later point in time, but that would make the timeline even more convoluted. I suspect it's more likely that this is just another shard that happens to be lightning-aspected, not the Twelfth.