All day events are only shown in the all day header. That makes it easy to miss the event, if it is more than just a reminder (like birthdays etc.). In Outlook the all day events are shown in the header, if I choose “shown as free”. As soon as I pick one of the other shown-as-options, I see the event across the entire day and also see its status – which does make a lot of sense! When I am on a business trip, on vacation, when there’s a holiday, I want the entire day to be visibly blocked. (confer https://forum.emclient.com/t/all-day-calendar-event/89904)

See four screenshots with all day events in Outlook - free - tentative - busy - out of office.