Why are multiple emails showing in a single thread in CRM?

This issue occurs because the CRM groups emails into threads using the Message ID, not the subject line. When you forward an email, even if you change the subject and recipients, the same Message ID is carried over. As a result, the CRM treats it as part of the same conversation and places it in a single thread.
To avoid this, instead of forwarding, you should send the message as a new email. This way, a new Message ID is generated, and the CRM will display it as a separate thread.