Old reminders are gone from macOS 10.14.6
According to Apple:

To use the new Reminders features in iOS 13—smart lists, a new quick toolbar, and more—you need to upgrade the reminders in your iCloud account.
Upgraded reminders aren't compatible with earlier versions of iOS and macOS. 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.

You will need to wait until you upgrade your Mac to Catalina before Reminders start to sync to your Mac again.