Hello meichen823,
Thank you for reaching out to Apple Support Communities.
We were able to find this:
What to expect after you upgrade
"Here's what you can expect after you upgrade your reminders:
- iCloud reminders are available on your other devices only if they're running the latest software.
- iCloud reminders created on a device that's running earlier software are visible only on other devices running earlier software. When you update a device to the latest software, new reminders on that device will be merged with the upgraded reminders and will be available on all updated devices.
- If you create new iCloud reminders on a secondary device that's running iOS 13 or later or macOS Catalina, but you haven't yet opened the new Reminders app on that device — for example, if you used Siri to create reminders — those reminders will be lost when you open the app for the first time. To avoid this, open the Reminders app once before you create any new reminders on an updated device.
- You can't see iCloud reminders in Outlook when using iCloud for Windows on your PC."
More details can be found here: Upgrading the Reminders app in iOS 13 or later
If the issue persists, can you provide a few more details? Which macOS version are you using with your Mac? Also, do you see an option to merge your reminders?
Cheers!
