The new version of Reminders for iPhone impressed us with its improvements in iOS 13.1. However, to sync Reminders between an iPhone and a Mac requires macOS 10.15, a.k.a. Catalina, which is released in the coming weeks.
"Upgraded reminders aren't compatible with earlier versions of iOS and macOS," Apple says on its support site. "If you upgrade your reminders on your iPhone with iOS 13, your iPad and Mac using the same iCloud account can't access your reminders until iPadOS and macOS 10.15 Catalina are available."
Apple suggests that you can view your reminders on a Mac or PC using the web interface of iCloud.com. However, people aren't delighted about the gap in service.
For now, users will have to wait for Catalina to arrive to get sync working again. We got an early glimpse of a beta version of Catalina if you want to know what's coming, and according to a leak from Apple's Danish website the update will be released on October 4th.