Calendar sync issues

Hello, Sier.

Thank you for using Apple Support Communities.

If you haven't done so already, make sure to restart your Mac Pro. A restart can typically resolve sync issues: Shut down or restart your Mac - Apple Support

Check to see what account you're using for Calendars and see if any of the changes are going through.

For iCloud sync issues, use this article: If your iCloud Contacts, Calendars, or Reminders won’t sync - Apple Support

"Refresh your calendars

You can refresh your calendars from the Calendars app:

1. Open the Calendar app.
2. From the View menu, choose Refresh Calendars.

Since the iCloud Birthdays calendar updates hourly, you might not see changes to birthdays in the Contacts or Calendars app for an hour.

Check your default Calendar, Reminders, and Contacts account

If you store and edit your events in iCloud instead of another place, like On My Mac, Exchange, or Google, then iCloud automatically updates your calendars. If you use multiple accounts on your Mac like iCloud, Gmail, and Yahoo, make sure that iCloud is your default Calendar account:

1. Open the Calendar app.
2. Choose Calendar > Preferences.
3. In the General tab, make sure that one of your iCloud calendars is the default.

If you store and edit your contacts in iCloud instead of another place, like On My Mac, Exchange, or Google, then iCloud automatically updates your contacts. If you use multiple accounts on your Mac like iCloud, Gmail, and Yahoo, make sure that iCloud is your default Contacts account:

1. Open the Contacts app.
2. Choose Contacts > Accounts.
3. Click the iCloud tab and make sure that you're signed in with your Apple ID. 

As long as you've turned on Reminders in System Preferences, your iCloud reminders appear on your Mac."

Best regards.

Thanks for getting back to us, Sier.

If the steps above didn't help with the issue, contact Apple Support to look into this further with you: Contact - Official Apple Support

Cheers!