On Thu, 5 Mar 2020 12:53:32 +0100, Claudius stated:

There are multiple bug reports about this and MS seems to have fixed it
once in Outlook 2010 but broke it again sometime later.

Outlook 2010 is ancient. Exactly where did you find these bug reports?
Have you tried submitting a new bug report? I assume you are using the
latest version of MS Outlook.

I think I was confusing it with the last working version. 2010 didn't have the issue. 2013 has/had it.

Here is the thread I bookmarked when trying to resolve this before giving up and shelving it:
https://social.msdn.microsoft.com/Forums/en-US/8eafe714-8a8c-44bc-9228-d6a68731494f/outlook-2013-behavior-as-an-imap-client

I think nothing has been fixed as Outlook still does APPEND.

Regards,

Claudius