Houdini 19.5.435
It appears that the occlusion node's (Karma) input "object scope" is not functional. When identifying geometry to exclude from occlusion in that field, nothing changes. Everything in the solaris scene is included in the occlusion, with no way of excluding objects from that occlusion as far as I can tell.

Unfortunately, VEX trace() scoping isn't implemented in Karma.

https://www.sidefx.com/faq/karma/ [www.sidefx.com]
I don't see it explicitly mentioned here, but I think no scoping implied by "simple trace" in the FAQ