Well, I don't have any screenshots, but I used the map editor to set up almost the exact same situation, and this time around they behaved like they should have. They fired at the advancing enemy rifle squads as soon as they stepped within the arc.
I was going to post all the screenshots of my test, but I ran it several times with the same result. For those of you curious, here is what the area looked like.
http://img65.imageshack.us/img65/9530/example1qp0.th.jpg' alt='example1qp0.th.jpg'>
Apparantly during my QB game when I first noticed this there was something else to it. I know my MG squads spotted the enemy themselves, because I could draw the line of sight tool from my MG out to the enemy, and it would tell me information about the contact.
The only thing I can think of at this point is that they had low ammo and I hadn't realised it. Or it's possible that during the action phase, they fired, shook up the enemy squads, they sneaked off, thereby losing contact, but still seeing them due to my other nearby squads.