Our group was doing 16th key and everything was pretty smooth until we encountered a roadblock before the bird boss. The third pack of birds before the boss was incomplete - one of the big birds was absent, so the start of the fight was not triggered and we got stuck on the platform with no way go progress further. No wind way to get back from the platform either, and the boss doesn’t come down. Leaving the dungeon with hearthstones and coming back didn’t make any change.
Our Demon hunter was able to locate the missing bird - it got stuck on one of the surrounding platforms. He was able to engage with the bird through trial and error, because it was pretty high up. After coming back to us to the main platform, the bird followed and we were finally able to kill it and start the boss fight.
Can’t post any screenshots as proof since it doesn’t let me to attach links.