Replies Not Posted To Sent Folder

I have an Outlook 365 problem.  Some, but not all, replies do not post to my Sent folder.

I performed the following experiment:

• I sent an E-mail to my forwarding address, which eventually was routed to the inbox of my live.com account.
• I replied to this E-mail using the following From addresses:

               My live.com default address

               My forwarding address

               My Spectrum address

• Only the E-mail with my forwarding address as the From address is posted to the Sent folder.

The only rules I have operate on the Inbox.  I don’t believe rules are a factor.

Does anyone have insight into why some forwarded E-mail, but not all forwarded E-mail, is posted to the Sent folder.  My current premise is that if the From address on the reply is different from the To address on the E-mail from which the reply is sent, then the reply doesn’t get posted to the Sent folder.  This premise has no logic other than observed test results.