The Ifill looks just like most any clearomizer with a replaceable head. They should not leak out of the mouthpiece, even when turned upside down. The way it could happen, if everything is screwed down snug, is if liquid is in the center tube, which can happen while filling, or as they get a couple weeks of use as seals shrink. One other possibility is that your leaking is caused by a loose head inside the clearo. This can come unscrewed a bit while taking off the tip, so you should make sure it is tight.
You'll need more clearos anyway, so get a couple of another brand to see if the Ifill ones are just not that good or you got one from a bad batch. Vision Ego clearos are a good quality brand. Good luck!