You either need to use update_field() to set defaults for every field when the theme is set up https://www.advancedcustomfields.com/resources/update_field/ or check the values and if nothing is returned then assume the default.
$value = get_field('my_options', 'option');
if ($value === NULL) {
$value = 'my-default-value';
}
I prefer the second method.