fontconfig fallback font for emoji doesn't work until I blacklist all other fonts

Question

I want to use UbuntuMono Nerd Font as my main terminal font but render emoji characters using Noto Color Emoji. I have configured LXTerminal to use font UbuntuMono Nerd Font.

I can see the primary and first fallback font files for this font with:

~ > fc-match "UbuntuMono Nerd Font" -s | head -n 2
Ubuntu Mono Nerd Font Complete.ttf: "UbuntuMono Nerd Font" "Regular"
fa-brands-400.ttf: "Font Awesome 6 Brands" "Regular"

To use Noto Color Emoji as the preferred fallback font for emoji characters not within UbuntuMono Nerd Font, I configure ~/.config/fontconfig/fonts.conf as follows:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <alias>
    <family>UbuntuMono Nerd Font</family>
    <prefer>
      <family>Noto Color Emoji</family>
    </prefer>
  </alias>
</fontconfig>

With this config, the output of fc-match indicates to me that this should work!

~ > fc-match "UbuntuMono Nerd Font" -s | head -n 2
Ubuntu Mono Nerd Font Complete.ttf: "UbuntuMono Nerd Font" "Regular"
NotoColorEmoji.ttf: "Noto Color Emoji" "Regular"

But it doesn't: '' is rendered within LXTerminal as a black & white smiley.

I have also tried using <match> elements:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <match target="pattern">
    <test name="lang" compare="eq">
      <string>und-zsye</string>
    </test>
    <edit name="family" mode="prepend_first" binding="strong">
      <string>Noto Color Emoji</string>
    </edit>
  </match>
</fontconfig>

The only way I can get the coloured '' to render is by blacklisting all other fonts. Why is a less precedent font overruling Noto Color Emoji when the output of fc-match is listing it as the preferred fallback?