I have experienced a serious bug with iOS voices a couple of times since I have updated.
It is hard to isolate when and how it happened, but I will try to give some clues:
-I am using iOS 11 in an iPhone 7.
-I have set my device's language to english, though I am from Spain and I read a lot of spanish content.
-I installed two additional voices: Siri female (UK), and spanish Monica, while my default voice is Siri female (US).
The weird bug is as follows:
I am reading some text in spanish, using the spanish voice. Suddenly I realise there is a little lag between my gestures and the voice output. Finally, when I change the voice language to another, English (Siri female uk, in my case), voiceover stops working completely. In fact, voiceover works, but there aren't any voice output.
I cannot read anything from the screen, till I decide to restart the device. After restarting, however, it doesn't speak neither. Minutes later, around 5 or so, I am not sure, voice speaks again, but not using my Siri voice, but the default english voice, and examinigng voiceover settings it seems that Siri female is not installed, and I must download it again, though I am afraid the system doesn't release the corresponding storage (I am not sure about this last point).
Have you ever heard about that?