Thanks for the details. Are you using the Outlook desktop app or Outlook on the web?
Outlook sorts Sent Items by To by design, and Microsoft removed the option to sort by From in the new Outlook. The supported way to separate shared mailbox replies is to turn on Save sent items in the Sent Items folder of this mailbox in the shared mailbox settings in Outlook on the web. This sends all replies from the shared address directly into the shared Sent folder, so you do not need to sort in your personal Sent folder.