Amendment to my previous answer: There IS code existing for AI to only target in a sight arc, but it is not implemented yet in TSR. We may implement it, but since it is untested on the FDS, it will not be anytime soon.
Stock Renegade uses line of sight checks for its infantry all the time (on the FDS and in single player) anything that blocks projectiles blocks AI line of sight; so unless you're using some scripts that actually go out of their way to find objects from the objects list and then just start shooting as soon as there is a clear line of fire TSR would also be making use of line of sight.
VIS hasnt worked with the reborn LE. Dunno if it works now.
Looks like that's something that will need looking into, thanks you for reporting it.
Some levels need to be regen'd every time you open it in the level, this is a common thing in ECW and on Bear Island, but in the past I have had a few maps where the vis stuck around from each time I loaded the map. I'm pretty sure it has something to do with how complex the maps vis is.