@JSWolf,
I have seen you refer to this Kobo 'bug' in various threads, but I have to confess that I don't know what you mean.
If I load a typical retail epub, containing an embedded serif font for the main body text, then I can see the publisher's font if I select 'Document Default' in the Kobo's select-a-font menu. This works even if the publisher has put the font-family declaration in body {...}.
Alternatively, if I prefer to use a Kobo internal font, or my own sideloaded font, for main body text then, again, I just select the one I want from the font menu.
Whichever of the above I choose, any extra special fonts embedded by the publisher to highlight selected passages of text also display correctly.
In what practical way is there a bug?
In real life, I've no idea why any Kobo owner would ever need to embed a main body text font as it's a lot less effort and disk space (not to mention more flexible), to just use the font menu.