> plain old epubs
Kobo devices ship two different rendering engines: the obsolete, appallingly bad Adobe Reader Mobile SDK (RMSDK), and their own, bespoke, WebKit-based renderer. By default, files with an .epub extension are rendered with RMSDK. Consequently, you'll get all sorts of weird issues. Some of the ones I've encountered: I've had cover images not displaying in the library, misbehaving internal links (footnotes), inability to override typefaces/text size.
Easy fix: rename the file to .kepub.epub[0].
Slightly more involved fix: run the file through Kepubify[1] first. This tool makes further tweaks to the books to enable a few other features, like remaining read time estimation.
[0]: https://github.com/kobolabs/epub-spec#sideloading-for-testin...