Check this setting first

Hey Siri, draft a message for XYZ” and then proceed to dictate your message, there’s a risk that Siri could interpret a long pause as an end of your command, causing the message to be sent prematurely.
To avoid this, you could:
- Start the command clearly: "Hey Siri, draft a message for XYZ."
- Keep a brief pause between sentences, but don’t leave a long gap, so Siri doesn’t misunderstand the pause as you finishing your message.
- Pause intentionally at the end of the message and confirm it's correct before saying something like “Send.”
You could also clarify the steps with Siri, for example:
"Hey Siri, draft a message for XYZ: 'Hi, how are you?' [Brief pause] 'Just checking in to see if you want to catch up later this week.' [Brief pause] 'Let me know!'" Then you can review it and say "Send" when ready.
This way, Siri will not accidentally send the message due to an unintentional pause.