Using Outlook for iOS in business environments for email and scheduling
I have the same issue with Outlook on iOS. I have been dealing with it for the past year. I was in contact with Microsoft's support team through the app. They had me delete and re-add the account which seemed to solve the issue for a day or two, and then the sent items on my iPhone never appear in IMAP sent folder on Outlook for desktop or my Outlook on my Macbook. This is very frustrating.
This isn't limited to one account. The behavior occurs with all 3 IMAP accounts I have configured on my phone.
The most they told me is that it's an issue with my SMTP server configuration, which I know cannot be the case as Outlook for desktop and other email client apps sync emails just fine.
There must a serious bug with Outlook for iOS.