When the voice drops in volume, starts to whisper, or starts to distort then this is most likely a stability issue or a voice issue. The prevalence is very dependent on the voice used, how wide the dynamic range is, if there is any low rumble, and in general just how well cloned it is and how good the original samples are.
For anything longer than a few hundred characters, we highly recommend using the Projects feature if you are encountering these types of issues. Not only will it be less prone to these issues, it will also allow you to regenerate specific paragraphs you are unhappy with, not the whole text.
You can find more information about troubleshooting such issues here and you can find our guidelines for voice cloning here.