Unfortunately, generative AI can be unpredictable at times. This unpredictability is present across all generative AI, and it is a challenging task to make them more predictable and controllable.
We have already made good progress in this department, especially with the release of the Projects feature for long-form content which we highly recommend that you use if you plan on doing multiple paragraphs of text instead of the Speech Synthesis.
When generating long-form content, the AI may, under certain circumstances, drift between accents or even between languages, especially if they are closely related. However, we have addressed most of these issues with the Projects feature, which allows you to generate very long-form content and appears to minimize most of these issues. If you do encounter one of these issues, you can simply regenerate a specific section that you're unhappy with, without the need to regenerate all paragraphs.
This issue is also more present in certain voices than others and you can read a bit more about how to hopefully avoid it here.