Set a different default #ACF value based on the post type

default-acf-value-by-post-type.php

<?php

add_filter('acf/load_field/key=YOUR_FIELD_KEY_HERE', function ($field) {
    if (get_post_type() == 'post') {
        $field['default_value'] = 'YOUR_DEFAULT_VALUE_HERE';
    }

    return $field;
});


// If the field type is a checkbox, then give it an array (even if it is a single value)

add_filter('acf/load_field/key=YOUR_FIELD_KEY_HERE', function ($field) {
    if (get_post_type() == 'post') {
        $field['default_value'] = ['choice1', 'choice3'];
    }
    return $field;
});