MAPI was working well with Outlook and then suddenly I noticed that all my recent emails (these had been read and/or replied to and forwarded) were appearing as unread with no other flags (replied/forwarded) on my phone and webmail only.

I first noticed this on my phone (iOS 13.3 using the Mail app with IMAP connection).

So, Outlook now shows the correct state of each email, but the emails almost appear to be new on the webmail and phone.

I've got logging enabled with Outlook, so let me know if you need anything.