Bug or by design? Spinebroken Nightguants.

I was gaunted the other day, and I used spinebreaker on it. It lay there (Stunned) but as soon as I left the room it was instantly back on my tail. Is this by design? I planned to break its back and jump into a lake, but I'd get two rooms away before it swooped back down on me. I broke its back 3 times too, so it wasn't a once off thing. Thanks.