I've seen this happen several times. It's an old bug. The below YouTube video has a good example, but there are many other configurations that create this problem.
https://www.youtube.com/watch?v=yKovD5VPUe4 (https://www.youtube.com/watch?v=yKovD5VPUe4)
One obvious way to debug is just to run a bunch of random checks across random points on the map, ensuring that A-can-shoot-B = B-can-shoot-A in all cases.
2x2 size turret has a similar problem.
(https://i.imgur.com/49xrkGT.jpg)
red is where turrets can attack colonists
Green is where colonists can attack without being attacked by turrets.
Moved to Mantis
https://ludeon.com/mantis/view.php?id=4258