There are few players remaining on this map
You are on a overflow map. Once the main map finishes the boss and players leave it starts eliminating extra maps. Just click no and finish the fight, get your chest then grab the buff for volunteering. Problem solved.
Why is this an issue? Click no and finish fight.
What’s up with the “empty zone” message when it is clear it is nowhere near empty?
The actual message is:
There are few players remaining on this map. Would you like to move to a more populated version of this same map?
That is an accurate description of what’s going on: there are few players on the map compared to its capacity and it’s an option, not a requirement, to move to another map that already has more players.
As other posters have suggested, finish the fight and then move: best of both worlds, since you get the reward + the buff.