Since the question pops up now and again here’s a cross reference to what to expect in this regard:
Especially this (emphasized by me):
I still haven’t found a way to deal with (or get used to) this particular design decision. It affects the classical part of my collection the most since it’s quite common to mix different sub genres on a classical album, even more so when it’s a box set focussing on a composer. In the latter case the way Roon handles genres becomes quite useless. Only the top level genre is what’s left to play with.
If Roon would let the user import a ROONTRACKTAG (or whatever this Roon custom tag would be called) there’d be at least a way to copy over track level genre assignments to Roon track level tags. Since Roon doesn’t show genres on track level it’s impossible to do something inside of Roon to somehow get around this design choice - or in certain use cases: limitation.