It's probably a coincidence but that's the one I had the most trouble with as well.
I think it dropped for me once before and I missed picking it up. It was when I first started doing the event and I hadn't yet realised most of the enemies don't drop any normal loot so any lootable corpses will be collectible items. I remember seeing a glowing corpse at the back of a mob in the opposite direction from where we were heading, and I assumed I'd loot it when the closer ones died and I looted them, but of course that didn't happen and by then we were moving on, so I missed it.
That fragment ended up being nearly the last one I got from that area (the last one was from one of the argent bosses) and I was trying to look out for it the entire time, which was distracting but I felt like it was necessary having possibly missed it once before.
Same problem, missing two splinters of weak enemies; got all the rest.
Especially in big groups - and isn't that almost always the case - it can drop on a mob without you having noticed.