The Stick behaviour keeps an object at a particular position relative to the target’s origin. When you flip horizontally, you aren’t moving the origin, just mirroring the image.
When you create the LineOfSight object, also link it to the Enemy object.
When you flip the enemy, you’ll also need to reposition the LineOfSight object linked to it.
After watching the video, and applying some changes, I finally did it. Though, there’s a problem, I can’t make them shoot. But that will be another story, thanks a lot, MrMen.