As of today, iOS 13 has already gotten two updates with the most recent 13.1.2 version aiming to iron out several bugs like Bluetooth and iCloud sync. Despite these efforts, iOS users are reporting that the Reminders app is not able to sync with Apple computers. Apple went ahead and released an official statement which explains that the sync feature will be restored with the official release of macOS 10.15 Catalina on October 4.
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 Catalina are available.
In the meantime, Apple suggests that users resort to the web version of iCloud which works on both Mac and PC. The Reminders app in iOS 13 is receiving some major functionality improvements with a new quick toolbar, smart lists, subtasks with attachments and reminders when messaging.