On Herobrine's Chamber {Lobby 18, and lobby 6 at least} the anvil fall has about a 2 second duration after the round is over. (About 1-3 anvils can fall afterwards} So, the bug is, when you need to make it to the target, if you time it correctly (or randomly) your enderpearl can hit a falling anvil and ruin that round for you. Not sure if this is a bug or not, but I figured I'd report it anyway. It's hard to capture videos because it happens so fast, and I've only seen it happen on L6 and Lobby 18.
I have seen it as well, but don't remember if it is specificity if it was just those lobbies. Perhaps the timing is just a bit off.
That's what I mean. The duration. And I'm not saying its SPECIFALLY those lobbies, but every time it has happened to me, it's happened in those lobbies.