Reminder completion does not sync across devices.

Hi wigtontw,

If you haven't done so yet, restart your devices. Although simple, this step can help correct some unexpected issues. It's a good idea to make sure your devices are running the latest OS versions since this can help with overall performance.

Is iCloud Reminders enabled on all of your devices? If not, this could be the cause of the behavior you've noticed. This article has more information: Set up iCloud for Reminders on all your devices.

"Set up iCloud for Reminders on your iPhone, iPad, or iPod touch

1. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.
2. Do one of the following:

• iOS 16, iPadOS 16, or later: Tap Show All, then turn on Reminders.
• iOS 15, iPadOS 15, or earlier: Turn on Reminders.

1. To view reminders stored in iCloud, open the Reminders app, then select a list in the iCloud section.

For more information on how to collaborate on reminder lists on your iPhone, iPad, or iPod touch, see any of the following:

• iPhone User Guide: Share lists and collaborate in Reminders on iPhone
• iPad User Guide: Share lists and collaborate in Reminders on iPad
• iPod touch User Guide (iOS 15 or earlier): Share and assign reminders on iPod touch

Set up iCloud for Reminders on your Mac

1. On your Mac, do one of the following:

• macOS Ventura: Choose Apple menu  > System Settings, click [your name] at the top of the sidebar, click iCloud on the right, then turn on Reminders.
• macOS 12 or earlier: Choose Apple menu  > System Preferences, click Apple ID, click iCloud, then select Reminders.

1. To view reminders stored in iCloud, open the Reminders app, then select a list in the iCloud section.

For more information on how to collaborate on reminder lists on your Mac, see Share a reminder list on Mac in the Reminders User Guide for Mac."

Best regards.