Well. it does actually happen as xenobots Distance3/4 are not dynamic, once the creature is on the desired distance u will stop moving, untill it gets too close again, or too far away. So trap 1 grim reaper with fire behind the stairs, put fire around it. (so it will stay in the unshootable place). pally comes in, targets it, but wont hit. so now u simply lure some other grims to the rp. bot will keep targetting that single "unshootable" grim. And die like a noobcake. Unless u've got proximitity on priority, but who does this on a rp/mage. this would mean your constantly target changing to the closest monster.
Ofcourse its not a bug. but it would be good if some AI could be added to the targetting.