I upgraded to iOS13 and updated the reminders app. It has pulled in all my lists EXCEPT for the main “Reminders” list. This list is visible in iCloud web app but not the reminders app on my phone.

I also had this problem.

I solved this problem using the following steps:

Turn on airplane mode.

Note: to backup your lists. (optional at your discretion) Backing up your iPhone will not help since the Reminders are gone.

Open a browser

Login iCloud

Click Reminders

Print each list as a backup Note: If the list is bigger than the screen, be sure to scroll down and print a second page.

After BACKUP:

On iPhone, go to Settings > select Apple ID, iCloud, iTunes & App Store

Select iCloud

Reminders > Toggle off Reminders

Delete all Reminders on iPhone > Yes

Now Reminders Toggle on

Close Settings

Open Reminder app

Your reminders should immediately be visible.

I hope this helps you also.

Respectfully, Candlia