Question: Why does the app change the luni-solar date at midnight?
Answer: The Calendar App is designed primarily to show parallel dates on the luni-solar calendar and the Gregorian calendar. The app changes both the luni-solar date and the Gregorian date at midnight in order to keep the two in sync. If it did not, the app would be in error much of the time.
For example, let's suppose that March 31 is New Moon Day in your location. At midnight on March 31, the app will change the Gregorian date to April 1. If the luni-solar date did not change at the same time, the app would be in error because it would highlight April 1 on the Gregorian calendar while still highlighting New Moon Day on the luni-solar calendar.