This is related to how canvas rendering works. For each canvas we basically generate one 'batch'. In the example you have sent there is a sprite renderer under a canvas. Currently this does not go through canvas rendering and as such will be either infront or behind depending on the canvas z / sprite z.

What we will do is expose the sorting layer on the canvas, but it's not a good solution for sprites ON a canvas... more for sorting canvases you may have in a 2d game.