System

OS: Void Linux
Version: alacritty 0.4.3
Linux/BSD: Wayland, Swaywm
Font: Fira Code, Apple Color Emoji

fontconfig

~/.config/fontconfig/fonts.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <alias>
    <family>foobar</family>
    <prefer>
      <family>Fira Code</family>
      <family>Apple Color Emoji</family>
     </prefer>
  </alias>
</fontconfig>

I have set font family to foobar in alacritty.yml, most of other emojis are displayed correctly except country flags emoji, I have also tried with Liberation Mono / DejaVu Sans + Noto Color Emoji, both can't display country flag character correctly.

Seems country flags has two characters, I guess this could be related, some flags such like Crossed Flags are rendered correctly.

List of flags emoji on Emojipedia

Screenshots

I have also tried alacritty on macOS Catalina, installed by using brew, got similar result.