Save file: https://drive.google.com/open?id=1i5XylNufe1vNbdSqTe7-BrtMVarjx5Vf
Columnists in question are "Ruspho" and "Areeloces". Related quest is "Phore's Mechanoids".
From Cynapse:
"The reason why we did this was to prevent cases where the buildings were destroyed but mechs were still attacking. The helpers leaving at that critical moment seemed unfair to players. I believe we set the help to leave after a day when all buildings were destroyed."
So not a bug - a feature! No, really. Thanks for reporting :)