Nevermind; Calibre on Windows has its own copy of those fonts. Kovid will know what to do. In the meantime check the Preferences for the editor and have it use Liberation; perhaps you changed it to something else.
I'm guessing that the font you're using is incomplete. I'm using Liberation and it shows those characters. Check and see if you can install the Liberation font package from Ubuntu. Otherwise manually install them from downloading from here: https://github.com/liberationfonts/l...fonts/releases