This is something that seems to start happening recently. 

My Microsoft 365 Exchange account uses an extended version of my default email - to login into Microsoft 365 I use freitasm@...microsoftonline.com but my default email is freitasm@geekzone.co.nz 

Emails sent from either the Outlook or web client show my default email as the sender. 

But emails sent from the Outlook Mobile (Android) show my account user id instead. Of course replies to that work, but it's not the preferred email address to show. 

My default email shows correctly but it is not what people receive:

This alias is also set as the primary email address in my Exchange settings.

Basically Outlook desktop and Outlook web correctly send messages from default email. Outlook Mobile seems to use the UPN as the sender instead.

Has anyone managed to change this?