For what it's worth, I can confirm Dio's problem - saw it with my own eyes.
I wish I could be more specific, but all I can say is that it's happened and it is a major problem.
I think I need concrete reproduction-steps. Ideally in video-form. In the save he attached the units aren't where they are on the screenshots. And if I move them there, they reaction-kill the enemy before he even can do anything. Maybe there's also sub-mods involved or specific option-settings. Ideally I'd like to have a zip with the options.cfg and all the relevant mods & submods.
I mean Speedislife has been streaming XCF with Brutal-OXCE since ages and neither him, me or any of his other viewers have reported anything the like.
Edit: Okay, I tried it again. I dropped Kaupo's weapon, so he can't kill the enemy. In this case Karim did not react 27,39,0 while the enemy was at 18,38,0. I switched both agents in the next attempt and in this case Karim reacted and Kaupo didn't. The enemy also couldn't attack the agent on 27,39,0. So it really seems that my initial hypothesis about the range being >9 is the issue here. It's right at the edge to be visible from 27,38,0 but not from 27,39,0.
Also when I don't enable night-vision, it is pitch-black there. On Dioxine's screenshot it is much more bright.
I'll now try with OXCE...
Screenshot is with OXCE 7.8.5 and as expected I didn't get a reaction either.
Edit 2: Tried with official 7.9.8 too and same result. Unit on 27,39,0 does not react to unit on 18,38,0. As it should be due to the darkness limiting vision to 9 tiles.
My current hypothesis as for why it supposedly has worked with that version is that there probably was some sort of always daytime mod enabled. This hypothesis is backed by how much brighter it is on Dioxine's screenshot than it is on the second one I just attached when I disable night-vision.