Took me so long to learn that the fix was to switch back to the old Outlook.
In my testing, the local IMAP client implementation quite frequently launches a DoS attack against your IMAP server. It'll send the same query requesting new mail messages in a tight loop, limited by the round-trip latency. But luckily, almost nobody uses IMAP via outlook because its so difficult to set up.
I've seen cases where people have it set up like that and it's so awfully slow. Minutes to display a single new message. That cloud brings absolutely zero user-benefit.
It's very frustrating and I can't think of a valid reason to configure things this way.
If you go into the "Outlook" menu in the app, there's a "Legacy Outlook" button, which relaunches outlook using a completely different binary. The two outlook implementations have different bugs and all sorts of different behaviour.
Outlook For Mac is free but "legacy outlook" requires a MS365 subscription for some reason.
Outlook is also not to be confused with Microsoft's "Web Outlook" client, available at outlook.live.com. It all seems totally insane.
This is Microsoft we're talking about, right?