Actually i dont think its limited to kera, as long as its not 1 hit kill, it will be right beside you even tohugh u may not see it (due to whatever reasons) if you stop attacking it for a sec, you might notice that it is actually quite near you, instead of far away as you see it when you are attacking. You will see your hp decrease by the damage dealt by kera but no numbers appear. Another possibility could be "invisible" mobs.