If this is a bug I personally don’t want it fixed but just confirmation I’m not crazy and maybe help some other players too.
In the current Ultron event I was the first in my alliance to do my clear against Ultron. I don’t have Miles rostered so I was only able to do the other subs along with all the Boss battles. When I finished the only open node was the Miles node after I did my final Ultron battle.
Later this afternoon I checked back to see how my alliance was doing and noticed we hit round 2. I also noticed I was able to do a Boss battle vs Ultron. Which I did. Now I just checked in again and we are on round 3 and once again I was able to do a Boss battle vs Ultron.
So it seems every time you reach a new round you automatically get a ‘free’ Boss battle even if you’ve cleared everything and are awaiting the next refresh. I only mentioned the Miles node in case you need to have 1 unfinished node in order to trigger this extra boss battle.
KGB