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.
I agree. Thi is is very frustrating. I’ve also been fighting it. The biggest issue is the same account with the same smtp settings work on other iOS devices. It’s a outlook iOS issue as it was working and after one of their updates stopped. And frankly their support as well I ruined as it may be is not worth the time as they only want you to delete and reinstall. I can’t do that as there are several thousand emails that would be lost.
I’ve turned to Spark email. Have had no issues and it’s a much better product. I’d come back if Microsoft gave a ****.