Set a default taxonomy #ACF value

_default acf taxonomy field.php

<?php // ignore - for gist formatting

// Specify your ACF field here
add_filter('acf/load_field/name=your_field_name', function ($field) {
    global $pagenow;

    // Set the default term ID
    $default_value = 'XXX';

    // Apply the default value only when creating a new post
    if ($pagenow == 'post-new.php') {
        $field['value'] = $default_value;
    }

    return $field;
});

default acf taxonomy field for specific custom post type.php

<?php // ignore - for gist formatting

// Specify your ACF field here
add_filter('acf/load_field/name=your_field_name', function ($field) {
    global $pagenow;

    // Set the default term ID
    $default_value = 'XXX';
    $target_post_type = 'your_post_type'; // Specify the target post type

    // Apply the default value only when creating a new post and only for the target CPT
    if ($pagenow == 'post-new.php') {
        if ((isset($_GET['post_type']) && $_GET['post_type'] === $target_post_type) ||
            (!isset($_GET['post_type']) && $target_post_type === 'post')) {
            $field['value'] = $default_value;
        }
    }

    return $field;
});