How to fix iPhone Calendar alert reverting to default?

I recently missed an appointment because my iPhone calendar's alert time switched to my custom default of "At time of event" instead of the 1 hour before I had chosen.

The iPhone has the most recent update and was fully turned off and on afterwards. I've noticed that it still reverts my alert to the default. I enter "1 hour before" using "Edit" and in a couple seconds after clicking "Done" it changes back to "at time of event."

I also tried creating a new calendar event (since the first one appears to be part of a series and automatically fills in some information), and the same thing happened. I need to be able to fix this or I risk missing appointments (or more likely I'll find a different app).

[Re-Titled by Moderator]