If an all-day event is resized to multiple days and then the user tries to resize the event back to one day, the event is not always resized.
The issue is occasionally reproducible and does not occur for all all-day events. The event is resized back only if the user tries to drag the mouse down and then resize. Here is a screencast - https://somup.com/cZeX6QpRlX.
It should be possible to resize back the all-day events back to one-day duration without moving the mouse down.