[rtiangha]

Like davidfor says, it has to do with the fallback fonts and the glyphs they support. Kobo pulls from the Kobo Tsukushi Mincho and the Kobo UD Kakugo fonts for symbols missing in serif or sans serif text when using Publisher Default but the book has no default font specified (and the Chinese font for those symbols too). Koreader uses Noto Serif CJK by default as its fallback font (I believe), which has wayy more symbols and better unicode support (it's Noto's reason for existence).

It would be nice if Kobo's fallback fonts had more glyph coverage (and bold, italic, and bold italic versions too), but the real blame should go to the book for failing to embed a font with the correct symbol support and assuming the e-reader would compensate.

To be fair, the stock fonts on Kobo are subsetted, so you may have better luck in sideloading a version of Georgia from a Windows PC or another e-reader like a Kindle or a Nook, which may have more glyphs (although it still wouldn't have full unicode support). I internally renamed Georgia Pro to Georgia from a Windows 10 PC and am using that instead because it has better pan-european support than regular Georgia (I'm also using a W1G version of Avenir Next for the same reason). I like the look of Georgia and it's part of Kobo's visual identity so I keep it; if I didn't, I'd probably take Noto Serif, internally rename it to Georgia and replace it (and replace Avenir Next with Noto Sans to keep things consistent, I guess), although I'm always concerned about RAM usage due to my large database and large number of fonts.