[MainCharacter]

Perhaps it's something like this...

Spoiler for Yasu:

Yasu's voice BEFORE they become a witch is unvoiced, but their voice after they become a witch is voiced. In a way, you can see the Witch Yasu as a Fantasy character. Once Yasu turns into a witch, we stop seeing the real Yasu in the story and instead are shown different representations for herself. The moment she "became a witch" is the moment we stopped actually seeing Yasu, the real person, in the VN.

So in a way, Yasu was never voiced. They only voiced Witch-Yasu, because it's a fantasy in Yasu's mind just like Beatrice, Clair, and Shannon. Perhaps they didn't want to voice the real Yasu to keep the character as vague as possible, but they felt that once Yasu became a witch, it was fine to give them a voice, as it could be a different voice from the real Yasu's voice anyway.