lost reminders after upgrading

I recently updated my iPhone 11 to iOS 14.2 and my iMac to Big Sur 11.0.1.

Today I turned on iCloud for Reminders to share them between these two devices. (I have not done this in the past.) Then I hit the blue upgrade button for Reminders on my iPhone and selected Merge (the only option other than Cancel). Now all my lists appear on both devices, but they seem to have reverted to some very old version. All of my newer lists have no items in them, and older lists have items that I know I marked as completed. There are even some lists that I know I deleted.

Reminders appears the same on iCloud.com.

Is there any way to restore my lists?