Using Classic Outlook on Windows for personal email, calendar, and contact management
In order to bring sent box items in conversation view in Outlook App, first go to Outlook Cloud Account and fix the setting there.
Log in to Outlook cloud account and follow these sets
View Settings --> Lay Out ->Manage Organization --> How do you want your message to be organized--> Select Show email group by Conversation
Once you complete above, come back to outlook app and restart the app with brand new session.
Now you should be able to view sent emails in conversation view.