When a reminder is created in the Reminder app and set for all day, the day shown for that reminder on the Reminder app main screen and in the Calendar app is always exactly one day prior to the day it was set for. I have turned automatic date and time off and on again several times, but this had no effect. It's correct if the reminder is set for today or tomorrow, but after that the app loses a day.Screenshot taken on May 21. "Do x" is set for 5/22, "Do y" is set for 5/23.
What calendar does Reminder sync with - Outlook, Google, Samsung's or? Does that calendar know your location and time zone, and does it match the location and time zone settings on the phone?
Outlook and Google's calendars used to work differently regarding all-day events and events lasting several days in Windows Phones some years ago. It never affected me in Northern Europe UTC+1 but it frustrated users in the USA.
So it may also depend on where in the world you are located?