There is a bug in the Calendar's Day and Week view that causes all day events to disappear when the window size changes. Adjusting either Outlook's window size (either the height or width) or the calendar size (by adjusting the width of the navigation pane or to-do bar) will show or hide the all day events.
Additionally, the "show more" arrow will not work and the scroll bar on the all day event section is disabled. Rather than expanding the all day event section so more events are visible, clicking on the arrow icon opens a new appointment form or returns an error.
Now you see the event:
Now you don't. Make the window just a little wider and the event disappears:
The fix? Reset the view. To do this, you can either open Outlook using the /cleanviews switch or just reset the Day/Week/Month view. Note that resetting the view is a temporary fix wile using /cleanviews has a more lasting effect but wipes out all custom views.
To reset the view on the folder, go to View, Current view, Customize Current view. Click Reset on the Current view dialog.