Supplementary font in kepubs does not work

[alex12500]

In kepub books that have pali or sanskrit words with diacritics, Noto Serif Devanagari is forced as supplemental font even though it doesn't have all of the glyphs needed. When selecting a side-loaded font as supplemental font that does have all of the glyphs needed, e.g. Noto Serif, it has no effect, the characters are still missing in case the reading font does not have the glyphs.
In addition, an annoying pop-up "Noto Serif Devanagari has been turned on as a supplemental font for this book" pops up sometimes even when the book is opened again and the supplemental font has already been turned on before.