Hello,
This is not a bug. As was stated in the announcement for this week, ninetails was updated. He now has increased health, defense, and resistance. We do not know exactly how he operates now, but it is presumed that he otherwise operates as before; i.e., server performance on ninetails will determine how strong he is for his next showing. This means that if you were unable to kill him this time, the next he comes up he will be a substantially lower level and you should have a much easier time killing him.

In the meantime, if you would like to increase your numbers we would suggest visiting the strategy section of the forum or the official discord for advice on building a team for this event mode if doing well in the mode is important to you. From what we've seen, most people have not previously bothered making a ninetails team since it was so easy before, so many people are unfamiliar with good ninetails teams and generally just bring a PVP team to the battle.
Thanks and good luck!