I notice this today too, I tryed to reproduce it succesfully.
Basicly when you try to dismante any fence it will show you which tile is connected to base of the fence. If zombie is standing on said tile it canot be targeted with guns (no outline) you can still attack it with melle.
That means bottom corner canot hit zombies outside fence, top corner canot hit zombies inside fence.
Interesting fact player build "Big pole fence" is working with no issue. Dunno if it has to do something with fact it cannot be dismantled (no base tile?)
