My old Kindle Paperwhite 2 apparently rendered the KFX version of the book correctly, after enabling the Publisher Fonts option, however, the web browser displayed fallback characters instead of Sinhala glyphs.
I also noticed that KindleGen does not support the Sinhala language code (si):
Code:
Error(prcgen):E23006: Language not recognized in metadata. The dc:Language field is mandatory.