bug about spawning mobs

[slaaayerrr]

some mobs have time to spawn.. like 2sec
if i use shadow pierser at start of their spawning - it can hit them but doesnt deal damage
so it supposed to be:
1) i piersing and deal damage to them; OR 2) i piersing and skip them because they still spawning (they shouldnt be targetable)
atm it is bug.

(its not only about shadow pierser, SP taken just as best example of bug)