When navigating a list of emails in a folder in the Mail app and previews are enabled, VoiceOver will on occasions speak the wrong preview. For example, you may place VoiceOver focus on an email received from Bert. Instead of VoiceOver speaking the body of that email as the preview, it might instead speak the body of an email received from Ernie.
There appears to be no pattern to predict when this behaviour will present itself or which email body will wrongly be spoken. In our experience, it's not been unusual for VoiceOver to speak the body of an email which isn't even in the same folder. For instance, speaking the body of an email that has previously been deleted. It's also not been unusual for this behavior to be present on multiple emails at the same time.
When you double -tap on the email, the correct and expected email body is displayed and spoken by VoiceOver.