Dan
I’d tried to upload my .ifc file exported from 2018, but on this forum, you can’t as it’s format is not allowed 
My nesting hierarchy in SU, as you saw maybe, is: level 1, or outmost group, is an ifcSite, inside which I have a subgroup (level 2) as ifcBuilding, then inside that, two subgroups (level 3), one for each ifcStorey, and inside those (level 4), the actual building elements like ifcWall, or ifcBeam etc…
When I say “isolate” I mean that in Bimsight, which I agree is horrid to look at, I mean that in the “objects” pane it identifies the sub-object (4 levels deep) like ifcWall, ifcColumn etc, but you can’t render invisible either building nor storey… Odly, it correctly identifies the ifcSite which you can render invisible…
I’ll try exporting with SU2016 on another machine when I get a moment… And yes, even from 2018 the objects owner is listed as Su2015 ! Bizarre…
Maybe there’s a wierd and wonderful methodology to respect, but I can’t work out the logic !
Thanks for spending time on this…