Sorry for replaying again, but I did a mistake replying to you.
It is actually normal, that the enemies are going away if they pass under the screen. Because the game should chalange you not to camp a lot and be more aggressiv. And at that point the energybar gets actually exciting, because as you get aggressiv (shoot, dash,etc.), than you loss energy and it forces you to camp again a bit. Thats the reason why I make the enemies dissapear, though the second part, where you say, that the enemies come immedietly towards you is definitly a bug.
I am sorry again for that long comment, but I hope you understand better what I actually was trying to do.