The official Twitter account just posted this:
— 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!