I encountered this bug on Steam playing on PC. I've downloaded Patch 3 and it does indeed fix the Hello-copter disappearing after bosses. When I was in free exploration mode however, I came across another bug related to this. Basically what I did is I let the Hello-copter go and then left the game, when I came back I only had the Hello-copter viewer in my hands and could not find the Hello-copter. I then attempted to do the old fix and uninstall and reinstall the DLC however all that did was get rid of the viewer as well. I now no longer have access to either of the Hello-copter items. When I deleted my data and restarted the game from the beginning the Hello-copter did indeed return to where it should be but I still shouldn't have to beat the game all over again in order to get the Hello-copter back. Still, Patch 3 is amazing, keep up the good work!
#Hello-copter Gone (post Patch 3)
noble idol