Level 5 EMP occasionally fails?

Somewhat new player here. I got the card for removing the level 5 wall, so now there's an EMP tracker that follows me around the level. Kinda fun, actually.

The weird thing is that twice now, I've been hit by it, but it doesn't work - I can keep shooting!

Is it a glitch, or because I have the emergency shield card, or Lucy Clover, or something else?