Reminders

It will spend some time converting your icloud reminder records to the new format and you will need your iphones and iPads to. convert ("Upgrade") reminders in ios13 in order to sync.

https://support.apple.com/en-us/HT210220

Apparently it can take some time to process of you have a lot of reminders.

I have a few hundred and it took only minutes.

I also believe it happens in the cloud