This tutorial has a lot of details on CanvasLayers.
CanvasLayers have their own
scale property that needs to be set to the same value as your camera. The CanvasLayers are not really "seen" by your camera.
A canvas layer is like a quick and dirty world of it's own. It has it's own viewport, so cameras in the base scene don't see the canvas layer at all. That canvas layer is rendered separate.
NOTE: CanvasLayers do have a few bugs right now (3.2), so you may have some frustration using them. Their scaling reports the wrong viewport size to it's children, so you can't bank on the anchors of a control working correctly. CanvasLayers also have a bug where "FollowViewport" doesn't work with translating mouse positions, so be wary of using that.