So, besides the bug, the embedded font might cause some problems too...
That seem to be the case, as far as my fast tests are good.
It is a pity because nobody here has tested the epub (yours and mine) with other ereaders than the Kobo & Sony, but we can assumed without too much risk that the Kobo is the culprit for now (btw with so many firmware updates, they are going to get rid of some old bugs one day
As the problem is related to Kobo, I suggest you to look here (https://www.mobileread.com/forums/sho...ans%3D0&page=4
# 48 and ss) on a regular basis and you may request information to Mrs_Often and davidfor: they are both very nice persons and if they can, they'll help you I'm sure.
If you want my "version" of your epub for playing with, I can leave it here or send it to you via a PM.