I have the same issue, new Mac, new installations of Outlook for Mac. It is now October 2024. I see that the first complaints about this DEFECT were lodged over a year ago.
WHY DOES MICROSOFT NOT FIX THIS?
The little red warning box does not remain visible for long. On a large screen it is easy to overlook it.
The effect is this: one can write an email, send it off, assume (how unreasonable!) that it was sent and then wait for a response, as I just did, for two weeks, on a time-sensitive project. Then, while I was looking for something else entirely, I find it in the drafts folder.
FIX IT.