It seems to me as more of an ACF issue than Gutenberg’s. Apparently if there’s an existing revision/autosave record in the DB of the post being previewed it will just return false when you try to retrieve the field using ACF’s functions. This is a workaround I’m currently testing:
function fix_post_id_on_preview($null, $post_id) {
if (is_preview()) {
return get_the_ID();
}
else {
$acf_post_id = isset($post_id->ID) ? $post_id->ID : $post_id;
if (!empty($acf_post_id)) {
return $acf_post_id;
}
else {
return $null;
}
}
}
add_filter( 'acf/pre_load_post_id', 'fix_post_id_on_preview', 10, 2 );
Another workaround would be to just use get_post_meta instead of ACF’s functions
