This is a known problem: Some Kobo fonts display bold instead italics in epubs. Kobo do not seem to be interested in fixing this, as they say we should be using kepubs instead. Also some custom fonts have a similar problem in epubs; this is usually because the font file names are wrong, or the font names in the files are wrong. Please see here for more info:
https://www.mobileread.com/forums/sho...d.php?t=211222.