Reminders app alert time keeps disappearing on iPhone 6s?

I use my phone's Reminders app frequently and have started having problems with the "Remind me on a day" option. Today I noticed that whenever I set a time and date for a reminder alert, it disappears after a few moments, leaving just the text I entered but no alert time (ie, a reminder to start dinner at 5:00pm on Tuesday will show up properly on the reminders home screen for a few seconds, and then the alert time will disappear, leaving just "Start Dinner"). Since the alert times keep disappearing, none of my reminders actually work at the moment. I also have weekly reminders that I've used for over a year but starting today they now show up as "(null), weekly" underneath the original reminder text.

I exited the app, restarted my phone and then did a hard reset, but nothing has changed the way the app is behaving. Creating a reminder through Siri just gives me the same problem. I haven't done anything out of the ordinary with my phone so I'm not sure why it's not working properly.

Check your app settings
Make sure that your iCloud Contacts, Calendars, and Reminders are set to appear in the Contacts or Calendars app:
Open the Contacts app and tap Groups in the upper-left corner. Make sure that All iCloud is selected.
Open the Calendar app and tap the Calendars tab. Make sure that All iCloud is selected.
Change how often your calendars and reminders update:
Tap Settings > Calendar or tap Settings > Reminders.
Tap Sync.
If All Events or All Reminders is selected, choose a specific timeframe instead, like Events or Reminders 1 Month Back. If a specific timeframe is selected, choose All Events or All Reminders instead.
Press the Home button.
Wait a few minutes. Open the Calendar app, tap the Calendars tab, and swipe down to refresh.
Since the iCloud Birthdays calendar updates daily, you might not see changes to birthdays in the Calendars app until the next day.