Dear @Alan Barker,
I hope you’re having a good day.
Thank you for the additional information.
Since you're using the New Outlook desktop app rather than a browser, I would recommend trying a few troubleshooting steps to see whether the issue is related to focus changes or keyboard shortcuts being triggered unexpectedly.
First, try disabling keyboard shortcuts in Outlook and see whether the behavior improves. If Outlook is temporarily losing focus from the reply to editor, keystrokes may be interpreted as commands such as Archive or Delete instead of normal text entry.

Next, please make sure Outlook is fully updated to the latest available version, as issues related to cursor movement, focus, and message list behavior have been reported in some Outlook builds.
I would also recommend testing on:
- A different computer, if available.
- A different mouse.
- Temporarily disconnecting any external input devices or keyboard-related utilities.
This will help determine whether the issue is specific to the Outlook installation, the device, or an input device interacting unexpectedly with the application.
Please let me know whether disabling keyboard shortcuts or testing on another device changes the behavior. That information will help narrow down the cause and determine the next appropriate step.
Kind regards.
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