I should also mention, in the vanilla game and videos on youtube of this mission being played, regardless of difficulty, have the original behavior of the counterattack triggering at 15% destruction. I made sure timed expansion was off, as that seemed like the natural cause, but the >5min counterattack persisted.
In the 2nd Aeon mission in supcom's campaign, your first objective is to destroy a Cybran city. According to the wiki, when the city is 15% destroyed, the enemy ACU will launch a counterattack. However, in less than 5 minutes after game start, the Cybran commander will launch the "counterattack" regardless of any offense. Is this intended?