I'm gonna express it differently; if an underdog is in the finals, its achievement. If half the horde of underdogs are in the finals with the same or more advantages with the star performers, then it is a bug.
As i see this, there have been several factors that caused earth (and possibly the other top teams) to stumble.
1. We lost some (more than one) key decks due to silly miscalculations. Obviously our rest of decks didn't work against the single deck that remained and kept opposing us. Take a look at round 5, and add a card to that immortal. Then look at matches after round 5, and use our immortal in some of the lost duels. We would have eliminated death, darkness and gravity for sure.
I don't know about light, though. But i guess, if we had more options, we would probably beat them too. This part wasn't a bug or caused by the event cards, though us getting darkness again an again as opponent surely helped us melt faster.
2. There is a spot in the vault size, where teams simply cannot win with all decks, this causes a collapse, after which they can stabilize at 5 or 6 decks, if they are lucky and salvage the right cards. It is probably not helpful to collapse among the last, because the other teams are all general/lt decks and your multiple decks could risk a total wipeout easily. This was further bittered by the nasty event card at the point of the last teams' collapses. It was almost sure to me that teams with 2 diverse decks will fare better than the top dogs, at that point.
I think, we would have still won the event if we don't mess it up at point #1, so i don't consider the whole outcome as a 'bug', But it is slightly embarrashing, that our top winrate (or 2nd place) was only enough for a 7th place.