After activating the third layer of sub-sub-categories, as explained in Sub-sub-categories - #7 by Michael_Uray, we are observing certain regressions of this (undocumented) feature.
Looking at the visual glitch described in Hide sub-sub-categories from /categories listing?, it seems the CSS for subcategories does not apply to sub-sub-categories equally, probably due to some kind of selector issue.
It would be really nice, if one could optionally hide sub-sub-categories from this view and have it use an unadapted display there.