Not sure if this is limited to 1.8, as I’ve never attempted this in 1.7.
This is a different to the previous issue I’ve raised here :- Unable to remove tags in 1.8 on Albums or Artists
Since the issue above, I’ve experimented using tags & Focus in tandem to achieve what I wanted, as part of the test, I added some Sub-genre to a tag.
However, if a incorrect sub-genre was added, even going in to the Tag, it seems impossible to remove the sub-genre.
When in a TAG, it’s normal to be able to click on a item, which will bring up a menu at the top of the screen, which allows you to edit the tag, and remove items. This doesn’t happen, when a sub genre has been added, as you are unable to right click to highlight (Win 10 PC).