In Post class, function save_attachments creates $this->attachment_ids array with keys prefixed with attachment prefix.
L821

                $this->attachment_ids[ $attachment_prefix . $attachment_id ] =
                    Api::set_prop( $attachment, 'post_id', $attachment_post_id );

In same class, function save_meta() tries to access saved attachment ids with only the attachment id as a key: L961

                    $attachment_post_id = $this->attachment_ids[ $value ] ?? '';

resulting missing $attachment_post_id and failed import. Adding prefix also to L961 seems to fix the issue:

                    $attachment_prefix = Settings::get('attachment_prefix')
                    $attachment_post_id = $this->attachment_ids[  $attachment_prefix  . $value ] ?? '';