I did some thinking and I believe I have the answer.

Espers, in the original form, bleed. They're animals, just like the people or monsters in the game.

Espers, in their magicite form, glow. They're already dead, so they can't bleed then. In fact, it's not even their body.

Come to think of it, Tritoch in the Narshe Cave was glowing, but he wasn't cut. So, maybe in the human world, they can glow, but they're still animals, so they can still bleed. I could be wrong on all this, but even if I am, take knowledge in this one point.

It's just a game and you're the first person in the world who actually cared about this.