The official Twitter account just posted this:
【ご報告】事後報告にはなりますが、2015年9月末にVoのランコがポリープ摘出手術を受けておりました。現在本人はリハビリ中ではありますが、冬コミに向けて豚乙女一同、新譜制作に取り組んで参ります。今後とも、何卒よろしくお願い致します。
— 豚乙女オフィシャル (@butaotome) November 2, 2015
“[Notice] It’s an old news, but at the end of September 2015 the vocalist Ranko had a polyp removal surgery. Currently, she is in rehabilitation, but all the Butaotome members will continue to work on the new release for Winter Comiket. From now on, please support us.”
Polyps are fluid-filled collections that form on the edge of a vocal cord. They are a result of vocal trauma without periods of rest. (more info)
This might explain why her voice was not that great during recent live performances. The rehabilitation period doesn’t last too long, so she will be good for C89! Go Ranko! Please don’t strain yourself too much!