It works for me on fabric. Note that the code governing this is

    private static final Pattern PLAYER_GHOST_REGEX = Pattern.compile("_\\d+");

    public static boolean isPlayerGhost(Player player) {
        Team team = player.getTeam();

        if (team == null) return false;

        return PLAYER_GHOST_REGEX.matcher(team.getName()).find();
    }

Perhaps a patch changes it on forge.

On fabric, it does not seem to work at all, but it might be Wynncraft related.

EDIT: Actually am I blind and is it working? Let me test with lower transparency.

Nevermind, it does actually work on Fabric. Let me get some screenshots of Forge.

Forge: