The Mail app comes with a built-in feature called the Conversation View that organizes related email messages into conversation threads. Conversation threading can help to reduce clutter in your inbox, but if you would like to display email responses as individual messages in the Messages panel, you can turn off this setting for your email accounts. Below are the instructions on how to disable the Conversation View in Windows 10 Mail app.
Open the Mail app and click on the Settings (Cog button) > Conversations:
You can enable/disable Conversation View for each of your email accounts (if multiple). Click on the Select an account drop down menu and choose the desired email account:
Toggle the Show messages grouped by conversations switch Off: