Images that are children of a world space canvas are not sorted according to hierarchy when their z is 0. To test this out, change the change the order of the "bg" and "check" images repeatedly, it makes no difference. When I change the z of any child to something other than 0, they start being sorted on hierarchy order, regardless of z. Also, interestingly enough, when you make the canvas size large (for e.g. such that it just fits into the camera frustum or exceeds it), the hierarchical...
World Space Canvas Rendering Issues
World Space Canvas Rendering Issues