I just double checked and there are no other objects with the same Z value.
Even when there are no other layers turned on, the moment I set a new a ortho cam to active, the layer disappears completely.
I dropped a new quad in the scene and applied the same texture and that quad doesn't have the same problem. I'm hoping that I can just rebuild that animation and the problem won't return. None the less, it's a strange problem and It would be nice to know what happened.
#XPression