All day events do not scroll in calendar - HOW?

All-day-events in a calendar do not scroll along with the other items in a day. A holiday tracking app we use posts holidays (vacations) into user’s calendars as all-day-events, but these scroll along with the other items in the day. I’d like to know how to keep them in place like the regular all-day-events

They have a CalendarDateTime of [date] 04:00:00 but this doesn’t seem to be the criteria that keeps them in place.

Any clues as to what field value needs to be incoporated in the document to use this behaviour?

regards