Marking email as unread issue in Outlook app

If I want to mark an email as unread in my Outlook app, I have to do it twice, because after the first time it changes back to the 'read' status (after a few seconds). The problems occurs on a OnePlus 5T phone running Android 8.0.0 and I haven't found anyone else online reporting this small but annoying issue. It looks like a bug to me, but maybe someone else has encountered this as well or has some ideas to solve this?