I have modeled cabinets with plastic laminate veneer. The panel substrate is showing through the veneer. I have noticed that in Sketchup if you cover a framework with a sheet material, if the material is less than about 1/4" thick the underlying framework shows through it. I have the material opacity set to 100.
Not a bug. It’s a limitation of your graphics card and OpenGL. What you are seeing is called Z-fighting. The various faces are so close together that your graphics card considers them coincident and is unable to determine which should be in front. As you zoom in you will see that the Z-fighting goes away as the distance to the camera reduces.
The usual fix is to tag the object behind the thin one and turn off the visibility for the tag so only the thin object is visible.
I think it’s a matter of proportion. If the model is only 10’ or so, a thin amount might render ok, but if the model is 100’s of feet like a site model, then it happens more. I think it’s a conscious choice made for the sake of speed of rendering the screen. You can send it to a rendering engine which doesn’t make the error, but it takes a lot more time to sort it out.
If I have anything “Z fighting” in my models, it does show up in Lumion too, so this is not limited to SU.
Here is that same model zoomed out and still no “Z fighting”:
It doesn’t create any value to the model itself per se, but if I do not include it then I would have to create a cutlist for that material manually instead of having OpenCutlist optimize and create a cutlist for me. But I am investigating the difference in time it takes to optimize sheet goods manually vs the time it takes to model all those parts.