I am a server owner for a Realm and we have lost 3 sniffers to date over a few months now. One day they are there, the next we log in to find their pens empty. My best guess is they are getting lost on chunk borders due to having a large hitbox. I will have to go in and try to chunk algin the next pen to test, but comsistently losing a mob that is so hard to get shouldn't be an issue like this.