Sending email as alias in office365?

Strange.  I decided to go with the DG option to allow me to send mail from the alias in an alternate domain.  Let's call them nick@primarydomain.com and nick@aliasdomain.com for these purposes.

This meant removing the nick@aliasdomain.com address from the list of addresses associated with my own mailbox, then creating the DG as you describe, assigning permissions etc.

In the DG, I set the display name and alias values to the same values as my own account, merely because I want my name to show the same way.

It didn't work!!!  Email shows as from nick@aliasdomain.com in sent folders on all my mail clients (which it didn't when the alias was attached to my mailbox directly).  But it arrived at the recipient (externally) as addressed from nick@primarydomain.com.  I also tried sending a mail from nick@aliasdomain.com to nick@primarydomain.com and it shows as from nick@primarydomain.com internally as well.

As a troubleshooting step, I set up a new DG for someoneelse@aliasdomain.com and tried sending from that address.  It worked no problem from Outlook Web, immediately.  Email arrived from someoneelse@aliascomain.com.

I tried changing the display name and alias on the original DG.  Didn't make any difference.

So is it a a matter of time?  That Exchange needs to propagate the change for a given recipient that was previously an alias directly against my mailbox and is now a DG?

Or is something is stuck in perpetuity because the address was previously an alias for my mailbox?

Any thoughts gratefully received!!!