The Follow Track constraint was constraining objects in a way that did not respect the original z-distance away from the camera; it seems like it's supposed to.
When the constraint copies the cameras z-axis it multiplies a z-axis unit vector with the camera matrix using mul_v3_m4v3, which actually gives the camera z-axis + its location.
This puts the constrained object in all sorts of weird places, especially if its already along the same line of projection.