Reminders not Syncing Across Mac and iPhone
I recently deleted all my completed Reminders on iPhone and Mac, due to the Reminder app response time being very slow. I had over 80,000 completed Reminders. After that, the open Reminders would not sync with the Cloud, on all devices. Searched for a solution and followed all the recommended steps, i.e. turn off iCloud sync, reboot, turn on iCloud sync, etc. Reminders started syncing, but all my Completed Reminders came back, thus making the response time of the app very slow again. Spent a long time with support and was assured that they would eventually sync after deleting completed Reminders again. However, they never did.