I think they may be unknown glyphs in the font you are using to read the epub. You could try embedding a font within your epub that contains the glyph. Chareink6 might do the job amongst others, and it’s available on MR.
https://www.mobileread.com/forums/sh...ight=Chareink6
Or maybe your .opf file in the epub is not encoding to utf8?
"Modify epub" in Calibre will let you do this painlessly.
The first line in mine is: <?xml version="1.0" encoding="utf-8"?>