Hi sorinfromtoronto,
From what you described in your original post, here is what we recommend: Upgrade your iCloud reminders on Mac - Apple Support
Next, we’d like to show you how to perform the recommended troubleshooting steps via the article above. Please click to open the link, then read through the steps in the section titled "Upgrade iCloud reminders."
1) On your Mac, make sure you’re signed in with your Apple ID.
2) On your Mac, choose Apple menu
> System Settings, then click [your name] at the top of the sidebar.
3) Click iCloud on the right, then make sure Reminders is turned on in the list of apps. (You may need to scroll down.)
4) In the Reminders app
on your Mac, click the Upgrade button in the Reminders sidebar, then follow the onscreen instructions.
If you don’t see an Upgrade button, you’re all set—your iCloud reminders are already upgraded.
Note: The upgrade process affects the reminders in your primary iCloud account only. Reminders in all other accounts, including secondary iCloud accounts on your device, remain unchanged.

The reason why we are recommending the steps above is because they will show you how to Upgrade your iCloud reminders on your Mac.
If you’re still experiencing the same issue, contact the community for more help.
Thanks for using the Apple Support Communities.
Cheers!