Hi
An annoying
known problem indeed.
I've updated to
2.5.1 (d5bb1c9bbd 4/22/13). Problem is still on.
I use i tags (not em tags) and real EPUBs ( not Kepub).
What's puzzling is that I have seem italics perfectly displayed on several books with these same side-loaded fonts, and then on a latter reading, the italics turned to faulty bold.
The fonts are the one I downloaded from official site. I did not modify anything inside. I just changed the suffix according to Kobo requests. Here is a joint example of one, initially perfectly good and which turned sour.
I think this problem could be repeated. Try it, wait some days, and try it again.