Video 1 -
Play the Elite Warriors mission, go inside Syrta and kill those soldiers. Try not going in front of their face or in the middle of their line of sight.
They won't see you.
I don't know what parameters were used for those soldiers in the single player campaign and if those were the cause of the bug.
The bug happens inside and outside buildings. I THINK (but this is only my thought) it's a bug happening when an AI is on a "cover" indexed position (behind a wall, in front of a window...)
I'm sorry I can't be more helpful.
Video 2 -
I just started near an house (the same one of the campaign for my comfort) in Syrta and I put two men inside of it with the editor and without any parameter: just double clicking inside the building.
I've tried to put the first AI in the entrance corridor, looking at the entrance door and the other one in front of a window inside the main room.
When I killed the first guy, I found the other soldier outside, prone, and he didn't react to my fire.
_________________________________
UPDATE: I've attached both a mission I've made where you can see the bug; and the .rpt file after playing that mission.
To reproduce:
1-Play the mission in the editor
2-WALK (not run) inside the building you're looking at
3-When you'll see the enemy, try shooting him. He'll go prone and won't do anything.