Supplement: The problem only occurs with "Xiaochen". You just need to use 'Xiaochen', use any text, try to synthesize multiple times, listen to the content, and you will understand what I mean by the question.
Thank you for the additional information. Based on your description, it seems that the issue is specific to the zh-CN-XiaochenNeural voice in Azure Speech Services.
I recommend reporting this issue to the Azure Speech Services support team. They will be able to investigate the issue further and provide a more targeted solution. You can report the issue by following these steps:
Go to the Azure portal and navigate to your Speech Services resource.
Click on the "Support + troubleshooting" tab.
Fill out the required information, including a detailed description of the issue and any steps you have taken to troubleshoot it.
Submit the support request.
The Azure Speech Services support team will review your request and provide assistance as soon as possible. In the meantime, you may want to try using a different voice in Azure Speech Services to see if it produces the desired results.