The reason for mentioning luinill is for a depiction of timelines. Both sides have been killing dragon consistently for almost a year now, bar the last two weeks. Hence, saying such circumstances have been present prior to that is more for an indication that it has been longer than a year.
I can't speak for how your clan was tackling basement fights prior to that period, but to prevent this exact issue from occurring as it was an outcome that we faced, we killed zombies weekly, despite the fact that humans were unable to compete for the basement itself. Sadly, we did not keep proof of this as it felt rather trivial. Rather we just adapted and dedicated time towards capture to guarantee access to basement.
As to the why this happened, again it could be from a pre-defined algorithm which we do not have sight of, and more than likely will remain that way.