Interesting that nobody has replied to this. I have found the same issue happening, though when it does, the enemy rarely fires or moves at all. Like you say, frustrating but easy money. I have reserved actions for numerous turns after visual and sensor contact, just to see if anything happens and when it's the enemy turn, they are highlighted then do nothing. After taking out a couple of mechs and moving forward to a closer engagement, what's left of the enemy start to act. I am running CAB V2.4, hesitating the update to V3.6, so maybe this problem has been addressed with the update.