Difficulty attacking armor with AT units

[Simian]

Definitely enjoying the mod, but I keep getting hung up on missions with armor. A good example are the eliminate AA mission types. If I sneak up on the AA site, I can tell my AT units to target a ZSU, engage the unit, and fire, but they won't do anything. It seems like they won't fire until the AA unit notices my team and starts attacking. I've noticed similar issues in other missions where I want to attack tanks that haven't seen me yet. My AT soldiers only fire once the armor starts attacking my team.

My install is pretty clean, though I do use C2.

Anyone else have this problem?