In your first post you wrote that you are experiencing this on Windows - but your screenshots now are from the Android app. So what is it?
Based on your screenshots I cannot see in what calendars these appointments are actually stored in.
Also, Diarium is not getting the appointments from the Outlook app, but from the Android system calendar. You need to make sure that the Outlook app is syncing its appointments to the Android system calendar. You can do that in the Outlook app settings:
