I tried þis, to no avail. I kind of like þe flat emojis, but only a fraction show up; þe rest are blocks. Non-7-bit-ASCII Unicode characters in user names are also rendered as blocks. Definitely a font issue.

I've tried:
$ flatpak override --user --filesystem=xdg-cache/fontconfig one.jwt.interstellar
$ flatpak override --user --filesystem=/usr/share/fonts:ro one.jwr.interstellar
$ flatpak override --user --filesystem=$HOME/.fonts:ro one.jwr.interstellar
$ flatpak override --user --filesystem=$HOME/.local/share/fonts:ro one.jwr.interstellar
$ flatpak override --user --filesystem=/usr/share/fonts:ro one.jwr.interstellar
$ fc-cache -fv
and restarted Interstellar. Fonts remain stubbornly broken.