Can’t upgrade Reminders app (in iOS 14.3)

When I open the Reminders app (which is synced to iCloud), a blue “Upgrade”

button is displayed above my default list. So far so good. When I click it though, the ‘spinner’ icon is displayed for a few seconds while it attempts to update, however then the upgrade button is displayed in the same spot again, and the app isn’t upgraded/changed (as none of the new features are added, such as sun-tasks etc.).

I’ve tried:

• Deleting all lists and reminders from the Reminders app in iCloud (on the web)
• Unsyncing and resyncing Reminders to iCloud
• Deleting and reinstalling the Reminders app
• Signing out of and back into iCloud
• Upgrading my MacBook Air to Big Sur and repeating all the same steps above

Note: when I try upgrade via my MacBook, the lists actually upgrade for a few seconds (e.g. lists that have been shared with me are now displayed as expected), but then it reverts to its previous pre-upgrade state.