The reading position is based on the current internal file plus the id of the span that includes the text at the start of the screen. If there is no span and hence no id, the kepub will usually reopen to the start of the chapter.
I haven't put this on my Kobo, but from a very quick look, the Japanese book appears to be one page per internal file. Which is effectively one page per chapter. Hence, it will appear to remember the correct page. Using the maximum font size. line spacing and margins, should push the text over multiple screens. If that happens, it should open to the beginning of the chapter rather than later in it.
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