When you go inside a spider/cave spider/creeper/enderman (or other custom mob visions if there are more) in spectator mode after going in third person, you will remain in third person with the vision on. If you then cycle to normal and back into third person, the vision is removed. Normally, the custom vision only works in first person, so the fact that it turns on if you go inside the mob with third person already on seems like a bug. There's a video attached to show this.