Recently i have been reported an issue where one of the user with SendAs permission to a shared mailbox tried to send an email From shared mailbox, the email’s sent are being stored in Sent Items folder of main user mailbox instead of sent items folder of shared mailbox.
Create a DWORD Entry DelegateSentItemsStyle and set the value to 1 in the registry under below Path.
Registry Path: HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences
(The x.0 placeholder represents your version of Office (16.0 = Office 2016, 15.0 = Office 2013, 14.0 = Office 2010).
Close and Re-Open the outlook to test by sending an email from the shared mailbox. if there are a number of machines on which you want to create the registry entry you can either use Active Directory Group Policies or Microsoft Intune Device Configuration Policies.