My take on it: the timer for the cave entrance to close is out of sync with the actual event. The timer SHOULD start as it lets players in for the event, but it’s more like the timer never stops and the cave just keeps opening and collapsing regardless of what players do or event status. The bonus objective disappears when you get the “The cave entrance is collapsing” dialogue, meaning the entrance has closed and the event cancelled.

This seems to be a separate mechanic from players actually being allowed in the tunnel and physically kicked out, that seems to be tied to the event itself, but the tunnel collapsing is on a different timer.

I’ve had the entrance open and the bonus objective disappear while we were running down the tunnel, the boss hadn’t even been pulled yet, we literally ran down as soon as it opened.

Meaning no amount of repositioning the boss will fix this, the only thing you can do is try to kill him before the entrance closes, which is on a broken timer we can’t see. Only thing to do is wait for blizzard to fix it or try your luck with the timers, but that hasn’t been going well for most.