Guys if you pay attention, you can see that Fossil Zombie hide itself when a plant trick is played... For that reason i don't think its an error, i think they do that with that intention. I mean, Fossil Head is untrickable because he is intelligent, its like when you use sunglasses, what's happens when the sun hide and starting to rain? You still use the sunglasses? Well, this depends of your intelligent too.. Of course, when plant trick is played he hide, but he don't hide when zombie trick is play because he wants the buff... Others zombies like cone zombie, yeah, he have a cone! But he is not untrickable, because he is a little dumb! , its the same of the plant, also i remember the first version of this game, pvz1, that plant appears and protects near plants of zombie attacks, but not protect when coffee bean is play on a plant... I think the developers maintain that behavior, and the parasol zombie, use the umbrella only when a enemy trick is play...
The problem here its the high strengh and health of Fossil Head, and well, we know he go nerf soon.