Basically, there is this frustrating artifact (which admittedly can be somewhat removed as stated in the post) still, even with Houdini 11.1. It was said to be fixed in later builds, but with a very basic scene (attached) it is very apparent. I find it a bit embarrassing TBH, that mantra would fail on such a simple render.
As the above post explains, it can be minimized by setting the 'noise level' on the mantra ROP to 1 (default of 0.05!). This can't be normal. Seems to be an issue with triangles mainly. Am I missing something?