Just stumbled upon this searching for similar solution.
You need to use the field key as reference, not the name, so it should be:
add_filter('acf/load_value/name=flexible', 'add_starting_repeater', 10, 3);
function add_starting_repeater($value, $post_id, $field) {
if ($value !== NULL) {
// $value will only be NULL on a new post
return $value;
}
// add default layouts
$value = array(
array(
'acf_fc_layout' => 'layout_1'
'field_key_XXX' => 'the content here'
),
array(
'acf_fc_layout' => 'layout_1'
'field_key_XXX' => 'the content here'
)
);
return $value;
}
