I took this screenshot in a hurry when I realized the game was over. At the very moment C and D was occupied by the allies and the next point opened, the game ended.
I think the mostly likely scenario is that the “25 min per objective” timer malfunctioned. When the next sector opened, the timer, which should have reset the time to 25 minutes, set the time to 0, immediately ending the game. I heard that when the 25min timer ends, both sides lose. Is it true? Maybe thats why the defending side got the defeat message.
P.S. I will report this issue in (Gaijin.net // Issues) as well.