Do multiparms within multiparms work in Houdini Engine for Maya? I've attached a picture of what the maya interface is showing when I try to do this (clicking the plus or minus buttons do nothing), and what it's supposed to look like in Houdini.
We generally advise users to not nest multiparms for this exact reason. You can sometimes get around the issue by re-syncing the asset, but that it tedious. I'd recommend that you refactor your HDA to avoid them if at all possible.