Thanks for your find, Boyuan.
Applying your patch (reformatted, attached) to
/usr/share/initramfs-tools/hooks/plymouth can temporarily fix this issue.
diff --git a/debian/local/plymouth.hook b/debian/local/plymouth.hook
index 7a64541..8824103 100644
--- a/debian/local/plymouth.hook
+++ b/debian/local/plymouth.hook
@@ -107,7 +107,7 @@ case "${THEME_NAME}" in
DEFAULT_FONT=$(fc-match -f %{file} 2> /dev/null)
[ ! -z "$DEFAULT_FONT" ] && copy_file font "$DEFAULT_FONT"
DEFAULT_MONOSPACE_FONT=$(fc-match -f %{file} monospace 2> /dev/null)
- [ ! -z "$DEFAULT_MONOSPACE_FONT" ] && copy_file font "$DEFAULT_MONOSPACE_FONT"
+ [ ! -z "$DEFAULT_MONOSPACE_FONT" ] && [ "$DEFAULT_MONOSPACE_FONT" != "$DEFAULT_FONT" ] && copy_file font "$DEFAULT_MONOSPACE_FONT"
[ ! -z "$DEFAULT_FONT" ] && ln -s "$DEFAULT_FONT" ${DESTDIR}/usr/share/fonts/Plymouth.ttf
[ ! -z "$DEFAULT_MONOSPACE_FONT" ] && ln -s "$DEFAULT_MONOSPACE_FONT" ${DESTDIR}/usr/share/fonts/Plymouth-monospace.ttf
